$510 left for my first PC build. Give me some suggestions.

Ok, what I have so far for $200. I only have $800 to spend.
CASE
PSU
HD
OD
I know I am getting XP Home OEM- $90.
What I need is CPU, GPU, MOBO, and RAM.

I want to let you know that I will be using this for medium gaming, the rest will be surfing. No Cad or anything. I will consider some configurations a few $ over $510, nothing ridiculous thought. I dont plan on getting another purchase until a few more weeks. I would like to wait as long as possible for the prices to fall on cpu, mobo, ram, etc. I will probaly get the OS and GPU for my next purchase. I have to but this stuff in 4 clumps, cant afford it all at once. This is my first build as well. I have done upgrades though.

I like this GPU. I dont plan on getting DirectX10 until next year, this will tie my over till then. Its also a great price.

I ask you to give me the best configuration and or suggestions for what I will use the PC for. I will consider both AMD and Intel. I only like Nvidia though. I would like a Motherboard that is upgradable, must have 4 dimm slots, 1 PCI-e slot is enough. I want to stick with AM2 or LGA775 sockets for upgrades.

Thanks for your time.