Need help choosing parts for gaming server

Wazzup Tek syndicate! 

My group of friends want to put together a new server mainly for Gmod, but also Minecraft, CS:GO, and maybe some websites. My friend is using a Pentium 4 with 4gb ddr2+ right now, and it lags pretty bad with more than 3 people. 

I've built some gaming PCs, but have no idea what we would need for a decent server. A total noob in this regard, any advice appreciated.

Budget: about $400, the less the better

 

Already have:

-250gb hdd

-Case

-380w 80+ bronze psu

Thanks guys

These are quality but low cost parts

http://pcpartpicker.com/p/CYLzGX

This will work for any game you can throw at it. You don't really need a lot of graphics power since you won't be playing on it.. you really just need the CPU power.

Highlights - 

  • Xeon processor (4 cores, and 8 threads)
  • 8gb of ram
  • Cheap graphics just for necessary adjustments and etc.

If you want to max the budget, or could afford a bit more, then I would just upgrade the graphics a bit incase anyone would actually want to play on the server PC.

I've got a spare GT 630... practically new... if you'll pay shipping, it's yours :P