what air compressor for paint?

You need a minimum of a 60 gallon tank and 5 HP for the motor. The paint spray guns are mostly low pressure (30-55 PSI) and high volume (10-20 CFM). Do a google search with http://www.google.com/search?hl=en&lr=&q=paint+spray+gun+air+Devilbiss&btnG=Search for more help.
http://www.spraygundepot.com/scripts/depot.exe?pgm=gunsdepotgate.bbx

http://www.autorefinishdevilbiss.com/paintcorecs.php

Go do some research, the Devilbiss site has the recommendations by the best paint spray gun maker.
Sample :

#66SD air cap Synthetic and acrylic enamels
Air Cap Pattern:

30-50 PSI inlet 7.9-12.1 CFM
Suction tips: 1.8 mm (66SS)
Order No.
Model No. 900345
46-6020
#66SK air cap Synthetic and acrylic enamels


More spray guns...

GTi® Millennium Gravity Feed HVLP Air Cap
#2000 air cap Base coats, clearcoats, primers, single
Air Cap Pattern:

4-10 PSI (air cap) 12.5-16.0 CFM @ 30 PSI inlet
Gravity Tips: 1.2 mm, 1.3 mm, 1.4 mm, 1.5 mm, 1.6 mm, 1.8 m
Order No.
Model No. 192136
GTI-407-2000

The 60 gallon, 5 HP compressor is a Home Depot special at about $400.

Too small a tank means that you run out of air volume and the pressure drops off. Then you have to wait for it to build up again. Not a good plan when your paint is curing inside the spray gun.

If the motor has limited HP, the compressor won't put out the volume needed to keep the tank pressure up. That means more waiting for the tank to fill up to the pressure needed to operate the spray gun.

I have the second gun pictured (the GTI Millenium). It's a nice gun and comes with multiple tips to help you adjust to different conditions. Recommended pressures are 20-25 psi for base and 25-30 psi for clear. I stay on the low end of that and use either a 1.3 or 1.4 tip, depending on the temperature (higher the temp, the quicker paint dries, and the quicker you'll need to put it down, meaning bigger tip). I'd look into oil free compressors, as these will give more pure air with less contamination. If not, get a good inline filter.

I'd do some research on google and on the internet, like jrickher said. A compressor for painting, especially in something like a car where you want to minimize the imperfections, is a pretty big investement. You want a good compressor where it can provide a steady, clean stream of air. Cheap compressors usually don't have the hp or the capacity to do this.

If you look at the specs of those two paint guns, try to find a compressor to match the needs of the gun. Ideally, you want your compressor to handle a little more than the gun needs, just in case. I did a quick search on sears.com and the cheapest i found was 1200 dollars.

You not only have to consider the pressure, which any compressor can deliver 30-55 PSI, but also the volume. Consider the flow available from a garden hose compared to a 3" firehose. At the same pressure, the fire hose puts more water. Even if you cut the pressure to the fire hose in half, it would still put out more water, .

Hear is a small thought that might help you, I have painted a bunch of cars in my garage at home and I learned to paint with a vacuum fed gun first then went to a gravity fed HVLP (high volume low pressure) the HVLP gun dumps much more paint and you need to move mush faster. I feel that they are easier to make mistakes with as well. I dont know how new you are into painting though so I may be preaching to the choir here. Or I could be crazy, so someone can correct me if I am wrong.