I'd say get a brand new eVGA, last time I checked it was $184 at zipzoomfly....plus you can do like I'm doing down the road and use their step up program. Send in the 6600GT when you have enough cash and pay the difference between the 6600GT and a 6800GT and you can upgrade to that
No clue, you'd have to check the terms of their program. You have to have EVERYTHING that came in the box though. I believe it might have said original packaging, plus everything it came with so they can resell it.
EDIT: Also you have to have your invoice from the original sale