I say go for the 6870. I don't know where people get their prices, but if you find a deal like http://slickdeals.net/forums/showthread.php?t=3293164
you can get a 6870 for $150 after rebate, and even sell the free games for another $25 back or so. I'm getting 290 MH/s with my 6870, and it only draws about 140 W (according to my kill-a-watt) above my baseline system, so you could probably even do two of them on your power supply.
Based on that 290Mh/s at $150, that's 1.933 Mh/s/$, and more than 2.07 Mh/s/watt, which is just about as good as I've seen for any card combination if you just want to do some basic mining with a regular PC.
Everybody else - please let me know if you're actually getting super deals on used 5XXX cards, I just haven't been able to find them.