Yeah, I was going for the 5850 it's slightly better than the GTX 460 IMO, but I think the 470 may be better than the 5850, that's why I can't decide between them.. I also don't know if the 470 would fit in the motherboard (size wize)
i'd go gtx460 and overclock but either the 5850 or gtx460 would both do high settings in most games at a reasonable resolution, what do you usually run your resolution at, and do you run sinle dual or triple monitors?
the rarely used part doens't reall matter, but really depends on the area you live in , nd who else is selling em, look on ebay/craiglist and price it competitivly, 8800gtx is old but hell its still better than some people (myself included) have so you might get a few bucks for it
as for single monitor with that resolution, either card gtx460 or hd5850 would be able to handle most anything out there on med-high settings