Your best bet for $300.00 would be the 7950 and your right on the limited memory bus of the 660 Ti. The only time that somewhat goes away is when you overclock that but at base clocks the 7950 is on top.
I have this Sapphire(pre BIOS) update for it.http://www.newegg.com/Product/Product.aspx?Item=N82E16814202003
Solid card. Performs extreamly well at its base clocks. however you can look towards the other Sapphire at $309.99 with the 850/925 clock.
As for a price for your 6870...I'm unsure. they sell new for ~$150. So an asking price of about $110-$120.