I built my current rig for a little over $800.
Chieftec server case, silver w/window + 450 watt PSU
Abit NF7-S mobo
AMD XP 2400+
512 mb Corsair XMS PC3200 (two 256 mb sticks)
40 gb Maxtor ATA133 HDD
ThermalTake Volcano 9 Cool Mod HSF
3 Antec Blue LED 80mm case fans
Rounded cables for CDRW and floppy