The 9800Pro o.c's well but you need to have a good core (R360 instead of R350) and good Memory (Hynix instead of the other ones). Ati tools usually tells you what you have in a documents generated for the card I think, you can check for yourself as well though, just look at the memory (if i has no heat-spreaders on it) and for the core tis a bit harder since you need to take off the cooler.
From what I read you must have 1 of the two ( both I have found only in my MSI 9800Pro tha clocked way beyond the XT speeds ).
But still XT speed aint bad considering the card costs almost half the price!
Also if oyu get a new cooler for it ( Artic cooling etx, -Just like the one HIS is putting on its cards ) you can go higher than that as well + stick some cooper heatspreaders on the memory modules as well and go even higher

cheap mods but effective if you desperately need more power!