the zalman fan is useless imo, the one u get with the cpu is really good. but it also depends the case u have (i have an antec p one p180)
about the ram, i'd go for 2gb too (i have 3gb now)
for the gc, i have a 7800gt. all my progs are working fine, i don't have any display problem and i can play (3d progs, not games
) at 1920x1440. i don't know what are the differences between a 7600gt a 7800gt and a 7900gt, but i'm very happy with my 7800gt
those cards are designed for games, then i think the differences aren't that big for 3d progs (open gl)