Well you'll need a sever motherboard and CPU, obviously. But try to get a LOT of RAM, and a few hard drives. Mainly what you need if a good CPU, possibly two CPUs, and a lot of RAM to experience a no-lag server.
The server we use to host all our Battlefield 2 games has four hard drives, four gigabytes of RAM, and four Pentium 2 slot 1 processors. And we never get lag. So just because it's old doesn't mean it doesn't perform well.