not really sure what you're asking. if you've got a big monitor capable of displaying larger resolutions, then the best picture possible on that monitor is done using the highest resolution in-game, so that there's no stretching taking place. playing a game at 768 when it's on a monitor capable of 1050, 1080, or 1200 on the vertical side means you'll get a picture that's far less clean. an 8800gtx is horribly inefficient with power, too. not really worth the money, all told.
if you've got a large display, it's worth investing the 160-180$ into a gtx 260 core 216 (don't bother with a stock 260). it's a huge performance difference - think 40% different than a first-gen 8800gtx.