Video card and Ram help

I have a Dell Dimension DM051 system(P4 3.4ghz, 1GB Ram) that came w/a ATI Radeon 256MB X600 Video card. I plan on upgrading the Ram and Video card. So far ive not been real happy with the Radeon card compaired to the 256mb nvidea card i had in my old system. Tiger Direct has the 7600 GT 256mb card for $150, is this a good card? and also i see there are 512mb cards out now, they have some same price range i was looking at, would it be better to go with a 512, like the 6600GT or 6800 512 cards? What would give better performance.And with the ram, i have 1gb, i can hold up to 3gb, do you still have to match the ram up on the newer systems like you had to years ago meaning that all had to me the same size in order to work?Thanks Ahmad