really depends dude, u could run a COD2 Server on a P3 800mhz.
But if you want something more up to date, i would be looking at:
1 or 2GB PC3200
Half Decent Motherboard. (DFI Nforce 4 Ultra D or MSI Neo4 Plat.. but anything would really do.)
If you never plan to play games on this this thing.. well get a PCI-e X300 or something cheap. (you wont be able to use an AGP card if the motherboard is PCI-e!)
80GB Seagate with NCQ or WD 10,000 RPM Raptor.
It really all depends on how many COD2 servers, and what else u want the server to do.