hey falco, i did the very same thing, added a pc 3200 stick with my 2 pc 2100 and had 768 MB of RAM
however after approx a year that pc 3200 stick broked down, for some unknown reason, system got unstable and shut off for no reason sometimes
1st i thought it was a virus but i couldnt find anything like that, so i figured ill try to unplug that stick and since that pc running fine.
well atleast so fine as 512 MB run hehe
Im not saying this will happen for you, it might been me damaged that stick when i increased the FSB to Overclock my CPU.
anyways its worth a shot
good luck
VanBuren
