The good thing is, you have left yourself with loads of room for upgrades.
Upgrade:
AMD Athlon 64bit 3700 64Bit (32Bit Compatible) CPU (1Mb 2nd level Cache) Socket 754 or higher.
1 Gb (2 x 512 Mb) Corsair TwinX, DDR500, PC4000PT, Cas 2
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256Mb X800 XT Platinum Edition (GDDR3, D-Sub, DVI, VIVO)
OR
256MB GeForce 6800 ULTRA DDR 3 Tv/DVI
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2x74Gb Western Digital Raptor Enterprise (10Krpm,8MB) on a RAID -0 configuration, using High Point Rocket RAID 1640 (4 channel SATA) controller.
For EXTREME overclockers use:
Yesico Fanless 420W PSU with TMS (Thermo Management System)
Vapochill Complete Extreme Edition - AMD (Socket A) Titanium with Side Window (FS-019-VP) £444 or $825 (USD)
The above cooling (+ cool looking case) will hit your CPU temperature down to -50 degrees centigrade allowing you to overclock your AMD Athlon by 1GHz or more.