PC upgrade time...thoughts?

Alright, I actually have very little knowledge of PC hardware. I just go with whatever people tell me to buy (with of course some little research).

What are your suggestions on upgrades (if I wanted to spend less than $800)? I'm looking at upgrading mainly memory, CPU (if needed), and GFX.

These are my specs currently:

AMD Athlon 64 3200+
Asus A8N-SLI mobo
1GB Corsair PC3200
Gigabyte GeForce 6600GT (PCI-e)
2 HDs, (40GB 10k and 300GB 7.2k)

I'm wondering how important it is to upgrade the CPU because that would indicate a mobo change. I also don't know the benefits of DDR2. I will definitely bump up the memory to maybe 2GB.


Thoughts?