If you want to keep the ram you have for an upgrade you'll want to get RAM that has the same speed (PC # and same DDR, # and type).
Eg. I have Corsair XMS 1GB (2x512MB) DDR 400 (PC 3200). Now if I want to upgrade I want RAM with DDR 400 and PC 3200, otherwise the slower RAM speed is the speed all of the RAM on your system will go, so there is no use spending more on faster RAM. So I could get, say... OCZ 2GB (2 x 1GB) DDR 400 (PC 3200) and it should work fine.
I would suggest though, that you stay in the same brand as your previous RAM so that you decrease the chance of there being a conflict.
So if I were to upgrade, I would probably get... Corsair XMS 2GB (2x1GB) DDR 400 (PC 3200).
If you don't want to take the chance of getting the wrong speed RAM or your current RAM is slower than you want, then I'd say just replace it.