Used 5870 or 5970 is your best bet.
Ignore used 6000 series cards.
7950/7970/7990 have the advantage of lower wattage but if you aren't paying for power that isn't much of a benefit.
Bad news is price on used 5870/5970 seems to just keep going up and up. Before Christmas 5970s were going to little over $300 (say $300 to $330 average on ebay). The price has just kept climbing since then.
Er wait ...
5870/5970s suck. Don't waste your money. Actually if you own some ebay them and buys some 7970s.
lol other than the small info about the 7000 series, this response seemed rather unrelated :<.
I've already got a 5870. Got it before the 5970 came out back in the days. Its done me good. Played the games I wanted and mines at a decent speed. Just,I want more lol. I'm looking at the 7970 because its available from NewEgg. I'd rather not buy used due to the already added stress. $400+ is too much to put in on a card that is already past its warranty and pushed further than it should for longer than it should. I also don't want to have to convert USD to BTC due to all the hassles of doing so (trust, complications, etc).
Also another reason I'm justifying another card, other than mining is so I can easily support 4 monitors. I'm not a dedicated miner. Just a hobbyist miner.