well it's all depend on your money though. hehe i you can, you can go with amd x2 am2 socket. the motherboard with major brand is a lot cheaper then if you go with core 2 duo system.
have you tried to look for x2 3600+ it's more less $120, and the motherboard (msi, biostar, asus, asrock, gigabyte) will vary around $60 to $109 here in indonesia.
as for the vga card, if you could, you can go with 7600gt, in my humble opinion, that card is the sweetpot for price and performance, but it's quiet expensive here as it's priced for around $220 (evga 7600gt ko 128bit 256mb ddr3).
btw what is your current specification? if you start from amd 64 microarchitecture, i guess there won't be much boost in performance as the amd 64 architecture is already fast at stock.
here is my list for microarchitecture's performance quick comparison: c2d > amd 64 fx/amd 64 x2 > amd 64/sempron/pentium d > pentium 4 > celeron.