For a 36" range, you need a 42" hood. If possible, you want it to be deep enough front-to-back so that it overlaps the front burners. As a general rule of thumb, you need about 90cfm per sqft of hood size. So, to stick with these dimensions, you'll get on the order of 650cfm. You can obviously go higher to compensate for various inefficiencies. So, let's say you aim for a 1000cfm hood. They are all variable speed, so if all you do is boil a pot of water, you can dial down the power. But if you cook greasy food, you want the full power.

At anything above 400cfm, many jurisdictions require that you install a make-up-air (MUA) system. If it gets cold where you live, you'll have to make sure it's a heated MUA system. That can get expensive. On the other hand, in other jurisdictions, they don't really care and you can get away with opening a window, whenever you use the hood. It's more labor-intensive, and it potentially is a serious safety issue, if you have other gas appliances. Carbon-monoxide backdraft is real.

If you vent straight out, you probably want a hood with an internal blower. If you vent somewhere a little further away, there are things you can do to reduce blower noise. In-line or external blowers are great, same as in-line noise mufflers.

As for brands, start looking at Modern-Aire, Vent-A-Hood, and Bluestar. There probably a couple of others, but this is a good starting point for high-quality and efficient hoods. Nothing is worse than buying a hood that makes a lot of noise and doesn't actually vent.

If you have a 36" gas stove then you need a 36" hood but going 42" wide will make your setup more efficient so that is something to consider. When finding out how much CFM you need the way we like to do it is buy calculating the total BTU's of your stove if you added all your burners together. So lets say for example your total is 50,000 BTU's then you would want at least 500CFM or more. If you require anymore than 600CFM then I would highly recommend using an 8" duct, as in order to product more than 600CFM efficiently you need an 8" duct and most hoods will come with a 6" so keep that in mind.

Another thing to look out for is what is on the inside of the hood, a lot of hoods may have plastic impellers and/or other parts inside the hood and putting something with plastic parts over a gas range is not recommended. I would also recommend looking for something with stainless steel filters as they are much more efficient than the standard mesh filters, can be cleaned in a dishwasher and will not brown and become more ineffective over time.

That's a good point. If you have the option, avoid any type of traditional filters. Instead get steel baffles or get "squirrel cage" blower. Much easier to clean and won't clog up.

For commentary on methods of air flow rate sizing, please read pages 8 et seq. of the Greenheck Guide for perspective.
http://www.greenheck.com/media/pdf/otherinfo/KVSApplDesign_catalog.pdf

BTU capability of ranges is only indirectly relevant to successful containment, where avoiding flow direction-reversing plume reflections at the baffles and hood structure is the real containment issue. (The capture issue is having a large enough hood aperture to begin with.)

I would suggest for residential use that one aim for the capability of achieving an actual 90 CFM per square foot of hood entry aperture. Typical blower fan curves and hood system pressure losses would call for selecting a roughly 1.5X greater rated CFM to achieve the desired actual CFM in the presence of the pressure losses. Because these losses vary widely, and analysis is dependent on often non-existent data, rough approximation is what we are left with.

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