i noticed a diff from 2gb on vista to 3gb. things ran a lot smoother. that was back on my old pentium d rig. on 4gb i love it. multi tasking powr! i can easily alt+tab out of crysis and in to wmp or firefox.
These parts come to $407 . It should have no problem with general usage and would be pretty good at gaming. I assumed they could reuse the optical drive(s) from their old system.
I would reiterate getting the motherboard I suggested the G41 is the budget version of the intel *4* series of northbridge chipsets. the P45 which I suggested is the standard enthusiast and second best. For the price of $8 I think its worth it.
I am running firefox, Itunes, msn, fallout 3 (minimised) plus background avg, spybot, Magic iso and internet dl manager (idle). Using 48% of 3gb ***** you 32 bit vista*.