Umm....the label on the memory sticks says clearly PC3200 DDR 512 MB so I implied they are DDR 1. Anyway, the motherboard memory slots have the same construction.
The slots are the same, I tried each memory stick in each slot. The top one is labeled DDR 1 and didn't work with any stick and the one under it is labeled DDR 2. It's weird that Windows XP only sees 512 MB RAM when I have both memories in, but when I use a hardware detection software it correctly registers 1024 MB RAM. Could this be a motherboard hardware problem?