9800 Pro - 380/340
9800 NP - 325/290
Dell 9800 TX - 325/290
The core can usually overclock very well but the memory is very limited (unless you happen to get a card with ram faster than is normally speced for your card like Sapphire would sometimes provide.).
The TX comes with 2.8NS memory according to some people over at rage3d so it should overclock really well! Should be able to reach ~350