Currently I run:
2GB DDR2 RAM.
Asus M2A-VM Motherboard
Athlon X64 5200+ CPU
160GB ATA Hard Drive
Some asus DVDROM (not burner).
Asus GTX 460 video card (brand new)
Silent aftermarket CPU cooler.
All inside of a Cooler Master Cavalier 3 case.
The above setup has been with me for about 4 years and has served me really well.
What I've learned with computer upgrades is that it's easy to think you don't "need" something when what you have works, then when you eventually do, you wonder how you could have gone so long without.