All I know is that my mom's system (asus a7n8x) will run 1gb of pc2100 fine. But as soon as I put a single stick of pc3200 it only recognizes the 3200 (even with 1gb of pc2100 in it). So, i'm not too sure if your having a similiar problem. Have you tried each stick by itself and have run it through memtest86 (
http://www.memtest86.com/)?
I'd try memtesting each stick individually, than both at the same time.