I was going to say something about that too.. But I'm no hardware expert and didn't want to step on advice-toes. Maybe you should ask this in the hardware section of this forum. I've been told that for bandwidth concerns; some boards need perfectly symetrical RAM installations... ie. not one 1GB and two 512mb sticks. It will work, but it's slower than 1gb total, installed symetrically (like 512 in slot one, and 512 in slot three (not in slots one & two)....
I really don't know for sure, but guys like Congo, Nick and CTJoyce in the hardware forum are true experts. I've watched and followed their advice and have yet to see them steer anyone wrong..