i would rather recommend building a brand new pc, because
-2gig is not enough for most games
-gpu is shitty
-vista is even worse...
-if you upgrade your gpu, you'll prolly need a better psu
-mobo might be necessary to upgrade.
=> 90% of the cost of a new pc are above components...
my recommendation: build a new pc
(look at some of the builds available on toms and ask for advice if you have chosen the parts.)