the #1 thing is to buy a better PSU
I would suggest at least 450W with at least 20A on the 12v rail
#2 I would actually suggest buying a 6600 GT for now, and wait for the new ATI and Nvidia cards to come out. but wait and see the benchmarks before you decide which of the new ones to get.
people are saying now "it looks like ATI will win"
I'm not saying they won't, but that's what people thought about the X800's
#3 I would suggest either an Athlon 64 3200+/3500+ Venice, or a 3700+ San Diego
the difference between the Venice and the San Diego: the San Diego has an extra 512kb L2 cache (stores instructions which allows the CPU to access them quicker than from RAM. the more, the faster generally)
a DFI NF4 Ultra-D motherboard (which allows for really good overclocking)
2x512MB G.Skill PC4800 DDRRAM (G.Skill RAM reaches really high speeds)
if not G.Skill, then 2x512MB OCZ PC4200
either Seagate or Western Digital hard drive(s)
the Raptor's (from WD) are 10,000RPM and if you put your OS and frequently accessed programs on it, can speed things up.
I would suggest a 36GB Raptor, and a bigger 7200RPM drive (for other misc storage)