Hi all, I have 3 RAM slots on my mobo (Asus A7V8X) currently with 3 x 256 pc2100 modules in. Last night I removed one 256 and replaced it with a 512mb module thus giving me 1Gb 2 x 256 & 1 x 512 This seemed to make the PC unhappy crashing out of explorer and various other weired stuff but it did run various programmes & games. The 512 is brand new and tested so no probs there. Is it a problem mixing RAM sizes??? CPU is an athlon 2.2+ running XP pro. Cheers.