I just replaced 768mb of pc2100 memory in my asus a7v8x-x mb with a 1gb stick of pc3200. This is the maximum and fastest memory it will support, everything should be working faster but instead it takes longer to long on to windows xp and to open programs. I was told by a guy at office depot this could be caused by the memory downgrading to 266mhz to match the fsb of my amd 2100+. He said everything was a lot more stable when the memory and processor fsb were the same. Is this true and if it isn't any ideas on why the upgrade is slowing my system down.

yes..this is very true! the memory has to match the proc. speed.. its a must! or it will run like crap!

You do need to have everything running at the same speed, otherwise you will get conflicts and system slowdowns.