I'd recommend getting a new Mobo, if you can find a decent one that will support your CPU. That way, you can get the full effect from an 8x AGP graphics card, but more importantly, it'll be at least a little upgradable (assuming you tend not to spend $100's on your machine) a little way into the future.
I've taken into consideration here that the memory of the type you're using is of limited stocks these days, and so getting expensive. (just go into a electronics store and ask for the price of a pair of EDORAM sticks, and you'll see what I mean. Ancient tech starts getting very expensive as stocks run out)
By contrast, DDR400 is incredibly cheap at the moment, and while it's not always advisable to mix RAM speeds, they usually clock down to the speed of the slowest stick, or it's otherwise possible to manually set in the BIOS. I'm lucky in that when I buy DDR400, I do it on an agreement with the store that if it doesn't clock down and run, I can take it back (he knows I won't blow it up lol)