lots and lots of ram!
My CS server (Hosts 10 games, 180 players):
Name: Potato
CPU: 2x AMD XP 1600
Motherboard: Tyan Tiger MP
Memory: 1024 MB of Crucial PC2100 Registered, DDR
Video Card: Matrox Millenium (MGA)
Hard Drive: IBM 75gxp 15.0 GB @ 7200 RPMS
Monitor: none
Storage Interface: IDE
Internet Provider: Broadband 10mb
Internet Connection Type: OC3+
Internet Connection Speed: 10mb
Operating System: Windows 2000
Other Components: (3) Netgear FA311
Of course, if you are running just one instance of hlds/q3/whatever, then a 450 will be just fine.
Keep in mind that cpu power is important, but memory bandwidth is too. Go for a 100mhz+ fsb if possible.