Play with vSync settings, especially in your drivers. Usually turning it on will help somewhat.
Conroe 2.4GHz 4MB Cache, 2GB 4-5-4 DDR2-800 Dualchannel, GeForce 7900 GT
LG 20.1" HDCP-compliant TFT-LCD, Logitech G15, X-Fi Platinum
SATAII WD 320GB 16MB Cache, SATA Raptor 74GB, PATA 250GB Caviar, Asus P5W DH