Here's a budget system I use in my office, mainly for office work, OK for casual gaming. My kids use a higher machine.
Athlon XP 2400+ (don't know if you can still get this)
Krypton C18G-400 (GF4 MX onboard)
256MB DDR400
80GB Seagate Barracuda
Liteon CD-writer
17" Samsung (non-flat)
2.1 Mercury SW-1480 speakers
Cabinet, MM Keyboard, optical mouse, etc.
<24k
-