Make sure you have the most up to date BIOS for your motherboard, and some of the settings in there can have major effects on performance. Go to
http://www.rojakpot.com/ for the most complete explanation of what all the settings mean. Also if it's an VIA-chip based AMD motherboard, you're gonna want to have the most up to date VIA 4-in-1 drivers installed (ver. 4.48, get 'em at
http://www.viaarena.com )