40-60gig HD, $30 video (to take load of system memory and CPU), a reasonable Athlon XP, 512-1G RAM
2x60GB in RAID 0 (it's not really that expensive)
Athlon 64 3000-3400
1GB Dual Channel RAM
daappleby's right, it doesn't take that much power to run a 20 person gamesever. Heck, my 1.2Ghz laptop can run a 35 person Battlefield listen server (non-dedecated) with full bots and not a lot of lag (practically nothing when run as dedicated)
EDIT: RAID will be very helpful if you ever find yourself on gigabit networks, and it ensures that the clients don't have to wait for the server to do anything. And even for the low budget server, I would highly recommend the gig of ram, as it would eliminate pagefile use, something that seems to cause a lot of my listen games to go framey.