As long as you're getting your RAM from reputable vendors (Crucial, OWC, Kingston etc.) you'll be fine. Most branded RAM come from the same factory anyway, like LCD screens. So look for Lifetime warranty etc. when making your decision. I bought mine from Crucial, they're pretty good.
figure ~$100 look and compare tigerdirect.com and Newegg. I just upgraded to 4gb of g skill ram from newegg.com for 79.99 shipped. My machine purrs.... I agree with above post....most branded RAM is fine. Just stick to a good name.
The prices have dropped through the floor - I think you can get 2 of the 2GB chips for around $112. ($56 each) These are the best you can get and Apple actually recommends them - you can ask any of the Genius's that work in the stores and they will agree that Crucial rocks!