Actually, it may not be the motherboard so much as the chipset used by that motherboard. Windows XP Home or Pro supports up to 2GB of memory max, but that shouldn't be a problem, as the 730S chipset on your motherboard only supports up to 1GB max.
What I'm thinking is a couple of things:
1.)The 512MB sticks don't want to work with each other; or
2.)There's something with the BIOS that either needs to be updated to a newer one, or is just not going to play nice with both sticks.