6800 HAS sm3.0 as already said and has the HDR thing....so the picture quality and support for future games is solid ....go with the 6800 GS..
First the X800XL has the HDR thing, just look at HL2, second picutre QUALITY will be about the same if not better on ATI (usually better quality integrated TMDSs and no texture shimmers at default settings (GF6 & 7 must be set to high quality). The SM3.0 advantage is limited, it's there, but not something I would use to decide on a card, only to break a tie, and that's pretty much the case with an XL vs GS, however check the game you like specifically because both cards have games they simply walk away with.
Rememeber the R9500 can do HDR, just not SM3.0/OpenEXR, so don't confuse the two, not that SM3.0 is worth enough to pick a card, just another tiping factor IMO.