I am in this same boat.
I have a new Aztek 470 and am currently testing out paint mixtures for the best result. I am using all acrylics, and either Tamiya or Model Masters. The Tamiya seems thicker but I thin them the same when going into the airbrush.
I pre mix a water bottle full of 50% Rubbing Alcohol/50% Water. Then from that bottle I mix 50/50 with the paint... or there abouts. I haven't had any real problems with it that much.
I definately recommend this article about airbrushing. It is extremely informative about spidering.. too much air... too much thinner, etc.
http://www.testors.com/hobbyist_guides.asp
Go there and read the airbrush guide. It is written by a guy that really knows what he is talking about. The problem is.. he doesn't give a straight answer.. he says just mess around with the mixture until you are happy with the results. He does have a bunch of tell tale signs with examples of what it looks like when you have errors and what the problem is. Its very informative.