You could probably Craigslist or Ebay a 2GB stick for under $10. Even check the forums here. That mobo caps at 2x2gb.
I wouldn't buy a new MB that's already outdated. You're better off saving till you can get a full combo (MB/CPU/RAM). If you can swing $200 there's quite a few options.
You guys made a point there, better making the whole switch later and just buy another 2gb ram.
Then for optimal performance I guess it should be exactly the same model as the one I have, right? even though the board supports up to ddr2 667, and the one I have is pc800?