help w/ motherboard and processor

i have a 250 gig SATA hd., 512 pciE card, all the paripherals. all i need is a motherboard, cpu and memory and i'm set. i could get my old desktop out but i'm thinking that i want a better processor because i had a 2.0 gig cpu (not dual core) and 2 1/2 ram and i had problems running a certain game (assassins creed) game at low levels. i want a better cpu and instead of doing that (getting my old one out) i could just get these few things and have something better for less. any ideas of what i should be looking for? i can spend up to 350-400 bucks. thanks for the help.

EDIT: i was given vista business(64)for free at school so i have that as well. i want vista so that is not an issue.