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).