--Case is entirely based on preference....barring that I'll give you my choice: Antec P180
--For the video card, since this is entirely based on budget, I assume you selected the 7800gt because of it's recent drop in price. If you're willing to wait an extra pay-day or pan-handle for a few days I'd grab the 7900gt because of its obscene performance and good price.
--The proc is a decent budget choice as long as you get the right core. If it were my choice though, I'd pick up an Opteron 144 or 146 and oc the HELL out of it; you'll be able to get it to 2.4 (conservatively speaking), maybe 2.6+, and thats a huge amount of money you'd save instead of going for a more expensive proc at those same clocks.
--For memory, if you aren't overclocking, or are willing to run at a divider, definetly buy the G.Skill 2x1 gig kit at P3200. That stuff is extremely inexpensive, but very good quality. If you are overclocking though, grab some of this stuff @ PC4000. Also a great deal for the speed.