The 5900 series can't hold a candle to the 9800 series when it comes to eye candy and aa/af, not to menion video quality is a litle worst with the 5900u's. Thus, if you have the money I'd suggest a 9800 pro, because the XT isn't worth the money at this point.. imo. Most 9800 pro's can do XT speeds anyhow, just need to check that the cooling is good enough in your case first.
the gv-n595u-gt is pre-overclocked at 500 or 520 mhz. out of the box.
some site had it OC'ed to 560ghz core/1.07ghz memory
with the stock cooler
it beats the xt in mostly every benchmark except 3dmark01SE
Just because it's OCed doesn't mean it will still beat the 9800 series in aa/af. aa and af have hardware dependent things on the board, which are independent of gpu speed. OCing will help aa and af a bit but I doubt it will be able to make up for the huge pitfall the 5900 series has compared to the 9800 series in aa/af and eye candy.