the cpu's.. they dont matter AS much as the video cards.. like the fx 57 is ridiculous... i've never met anyone with that kind of money. one of my friend's have the x2 amd cores and he said it's fairly decent. with the fx series, you'll be able to focus more attention on ONE program and the cpu will juice that one and that one only program up.
computer software writers have been recently told to make games in multithread processing methods.. so dual cores will be necessary some time in the future
the gfx card.. go with ur heart rofl. EVERYONE fights for what's better, and what's not.. since you've mentioned you're not the spend the million dollar type.. i'd suggest going for a 6800gt or x850xt. those are mid ranged video cards.. some may say HIGH, i say medium
give me ur budget on how much you want to spend on ur computer.. and do you want to start fresh from a new motherboard? or just do an upgrade.. you can only upgrade if you have a pretty recent motherboard with the chips these days.
finally, no you dont, you dont know that you want a 7800gt
. do some more homework :P