Well if you are just looking for a performance increase and do not want to spend a lot of money - then consider a memory upgrade. I can't advise you any further because I don't know your system configuration. But RAM upgrades make a massive difference in games. Also how many start-up programs you have would make a big difference.
I personally only have Norton Antivirus and Firewall 2004 loading up on my system. But other programs like MSN, IRC, ICQ etc wouldn't make a massive difference (unless you have them all loaded at the same time).
[size=0.5]Full Aluminium ThermalTake Case
AMD Athlon XP 3200+ *400 FSB* Barton (2.2GHz)
ThermalTake Volcano 11+ Xaser Edition Air Cooling
1024MB Corsair TwinX XMS 3200 (Dual Channel 2x512) 2-3-2-6
256Mb XFX 7800 GS EXTREME
2x WD Raptors (10krpm 8MB Cache) 36.7GB using RAID -0
Video Logic SonicXplosion 5.1 Soundcard[/size]