Actually, this is what I found in the motherboard manual (I thought I'd already posted this, must be the forum being sickly earlier today):
PC2100 (DDR266) for 3 DDR DIMM slots , Max. 3GB
PC2700 (DDR333) for 2 DDR DIMM slots , Max. 2GB
PC3200 (DDR400) for 1 DDR DIMM slots , Max. 1GB
(bolds by me)
I'm not entirely sure how to interpret that, but I understand it as the mobo only being capable of handling one DDR400 stick at a time, and that one can only be at a max of 1GB. I've been using THREE DDR400 sticks at a total of 1.5GB. No wonder it's been unable to sustain that...
I didn't build this thing myself; it was just one of those fairly cheap pre-made packages, and I suppose the first things they make cheaper are stuff like mobo and PS, since they don't expect people who buy them to do any upgrading anyway. Well, I did some upgrading recently and it obviously got too much for the heart of the system to keeo up with.