you'd only be disappointed in benchmarking. while the x800p can beat the 6800gt in some settings with aa/af, overall the gt wins more than the pro.
the difference in gameplay at the same settings is not noticeable however, as both cards play pretty much everything well. overall tho, i'd say the gt is marginally the better performer, and at this point i'd say sm3 is a better feature than 3dc & temporal aa, but again, you're not going to notice which card you're playing on unless you're running benchmarks as there's no difference in playability.
doom3 may make a difference, but we'll have to wait and see if nv's ultrashadowII will make a noticeable difference, as d3 is capped at 60fps anyway.