HD 6870 due to the warranty still being intact; otherwise, the 6850 as GaiaHunter said, or paying more for a new 6870.
Most gamers don't let GPUs live their full life [not from what I've seen], so even though you might like the feeling of buying a new card, the 6870 is the better option - as it still has the full, second user's warranty.
P.S. Some people say CF it later - I've tried CF in the past [two 5770s] and didn't like it at all. I personally recommend the best price/performance [in the gaming sector] single GPU cards for upgrading each time.