Windows only will use what it needs. Since you have so much it apparently only needs 20% of it. Gaming will tax the CPU and video card, well sometimes, but only rarely uses system RAM (mainly when yer GPU sucks total ass). But with a good video card it will barely ever tap into yer RAM resources. I only use 256MB of PC2100 DDR (OC'ed to 290MHz) and have a GF4 Ti4200 (OC'ed to 4600) and I never get slow downs, and XP only uses like 80-85% of my mem and I play max payne, GTA 3, etc at very agressive settings.