I have an old TOL Sony receiver that sounds great and can synthesize surround sound into three speakers. I would guess that that capability is not all that unusual nowadays. But it will require a center-channel speaker.

Decent centers aren't all that expensive, though it's best if you match it to at least the brand of front speakers you're using (if not a center designed specifically to complement your existing Jamos). I think you could buy a new receiver and center-channel speaker for around $500. Or less, depending on what other features you want and how much you're willing to shop. You might want to check out ebaY if you're comfortable with buying that way.

Nah, you don't need a center. There are many receivers out there that can simulate a surround experience with a stereo pair, they just create a slight delay in some of the sound, and knock it out of phase. If the voice is coming out of both speakers simultaneously, you'll create what is known as a 'Phantom Speaker' in the center.

Yamaha came out with a speaker that has about 60 drivers in it. This one speaker can simulate a surround sound system and I've read a lot of good things about it.

What really makes a system is a sub though. If you don't have a sub, you won't have a very good 'movie' experience.