homeowner

what size is existing system? other than age, what if any are the problems?

you will need an outside HP condenser and inside air handler. it would be a waste of time to replace just the air handler as you want a matching system.

I always recommend a thorough ductwork inspection.

you do want to take advantage of the Fed Govt tax credit up to $1500.

these are the minimum specs that I recommend.

15 SEER, 12.5 EER, 9 HSPF
best matching VS air handler
full BTUs in both cooling and heating for rated size
R-410a(Puron) refrigerant
new refrigerant lineset
scroll compressor
electronic demand defrost preferred
thermistat with "dehumidify on demand" feature
staged backup heat strips

my personal favorites are Carrier Performance 15 "H" mdl and Bryant Preferred 15 "H" model.

I will not comment on price as this can vary from the area you live, size, brand, and model.

IMO
Good Luck!

Thank you, tigerdunes! This will be very helpful as I start getting estimates.

SEER's overrated and over-promoted. It's only useful for selling boxes. Even in Florida, high-SEER systems don't make financial sense. The difference from a "16 SEER" and "19 SEER" system worked out to $0.10/day savings for my house, including forecasted energy rate increases. Extrapolate that out and $365 saved in 10 years isn't going to cover the price difference.

Other problems with SEER are how easily the # can be manipulated without any benefit to comfort. Just running the blower 30 seconds after the compressor shuts down will get a manufacturer about 0.5 SEER but will increase the humidity inside your home during cooling. Similarly, SEER doesn't reward moisture removal. Without a variable speed blower or intentionally mis-matching equipment, you'll find AC systems made 30 years ago will remove more humidity than those made today. If you insist on comparing #s, use EER instead. It's just a straight calculation of BTUs per watt. FWIW commercial equipment only specs out in EER for a reason.

Do you have any rooms which are warmer/cooler than others? Any place where noise from the HVAC system is the loudest? Any where it's the most objectionable? Contrary to what many contractors will say, it IS possible to fit a silent HVAC system in even a small space.
Any particular rooms where the system puts out a bothersome draft?
Does the system take too long to cool/heat?
Do you travel frequently? Do/would you use temperature set-backs when away from the house? Sometimes it does make sense to slightly oversize.
What temperatures do you prefer in the summer and winter?
How long do you expect to stay in your current house? No use putting in a top-of-the-line system if you're selling the house next year. Chances are the future buyers won't appreciate it.

Start with those questions. Make sure the contractors are aware of any of these issues.

Next, start calling contractors. Lots of them. Get a minimum of 5 bids. Why at least 5? Chances are you'll get a low bid, high bid, and everyone else will fall somewhere int he middle. Then you'll have a clear idea of what the going rate for the job REALLY is without getting a shoddy install or paying too much. The more the merrier. I called 8 contractors when it was time to redo my home's AC. Only 7 showed up, only 5 gave bids.

Next, throw out any bid by a contractor who doesn't do a heat load calculation. This will require that they measure rooms and windows. Rules of thumb don't apply, even on tract homes. If your house has large windows exposed to the south and the house next to you has their large windows exposed to the north, there's a good chance the heat load will be different. I've done enough remodels to be able to ballpark things, but there are always surprises. Even two identical condos in the same building can have different requirements.

Going through tigerdune's list:
*SEER ratings -- who cares? If high SEER doesn't make sense in Florida, it's not going to make a difference anywhere. Don't count them out entirely as dealer & utility incentives can sometimes make it worthwhile.

*Matching indoor & outdoor units -- you've got a 20 year old unit. Change both at the same time. It's close to impossible to calculate out the system performance of an older component with new components added in.
*R410a (Puron) - This is the new federally-mandated refrigerant. You might as well bite the bullet (probably an extra $100 or so) and get equipment that uses the newer refrigerant. R-22 (Freon) production is being cut severely in Jan 2010, completely in Jan 2020. Chances are if someone's pushing R-22 units, they're older surplus units that have been sitting in the warehouse for awhile. NOTE: R-410a is VERY hydrophilic (absorbs water). Don't let them install/work on your system when it's raining outside. Even if they pull a vacuum it's still not wise. Water doesn't compress well and your compressor won't like it.

*new refrigerant lineset -- Yep, if at all possible, replace the lineset. Chances are the older lineset will be too small and at the very least, it'll be contaminated with the oil used in R-22. R410a uses a different oil. There's methods to clean a lineset out for reuse, but it's best to save that as a last resort.
*scroll compressor -- I disagree here. I'm perfectly happy with a reciprocating compressor or scroll compressor. Scrolls have fewer moving parts and are quieter, while reciprocating can take more abuse. On multi-stage systems, reciprocating compressors will have better dehumidification. With a compressor blanket, a higher-end reciprocating compressor will have the same noise level as a scroll.
*Electronic demand defrost -- Make sure whatever you're buying has this. This is where the cheaper brands skimp. There's two types of defrost boards used in residential heat pumps. Timer based (defrost every xx minutes) and demand-based (defrost only if coil has ice on it). It obviously doesn't make sense to defrost when there's no ice to defrost, but the cheaper brands will, firing up the heat strips and running the heat pump at high stage each time they do. Timer-based controls also don't know when they've removed any ice so they'll run longer than need be as well. For comparison, the Goodman heat pump that came with my place was timer-based. Every 60 minutes it would defrost for ~7 minutes. For comparison, the Carrier Infinity system that replaced it only started defrosting when the temperatures were down in the 30s & 20s, probably around 2 hrs between defrosts, about 4 mins for a defrost cycle.
Thermostat w/dehumidify on demand -- I'd take it one step further, Honeywell IAQ thermostat *or* Carrier Infinity. Controls DO make a difference on how comfortable the interior of the home is. After all, it's why we use them instead of a big on/off switch on the wall. For best performance get a variable speed blower that has some dehumidification logic. Most do these days.
Staged backup heat strips: This is just having multiple heat strips inside an air handler/fan-coil that can be switched on individually. This depends on having a thermostat that's capable of controlling them.

For split-system heat pumps, I prefer Carrier Infinity/Bryant Evolution systems. There still isn't another residential system out there with controls as advanced as Carrier/Bryant's. To give you an idea, the indoor blower speed is determined by the outdoor temperature speed to make sure the air coming out of the vents is warm even on the coldest days. Good low-temperature output on the unit too. Even with temperatures in the low 20s the backup heat strips never came on. By the time you get done with rebates/incentives/credits, you can usually get one of these within a few hundred dollars of the Performance/Preferred systems and there's a big difference between the two in terms of performance.