I have a 6600GT - never had any kind of framerate issues to the point it affects play. Certain locations like Org could get quite choppy soon after zoning in, but that seems to have been a memory/load time issue. Going from half to a full gig of ram meant i can walk and look around pretty much immediately after hearthing. Haven't done MC or anything, obviously... but if the card could handle the hundreds of people congregated in org during winter feast, half of them in snowball wars, with a fairly smooth framerate... and that was back before the memory upgrade.
Last I checked pricewatch had someone selling this card for about 70-80 bucks.