I Need Help With Upgrade

Hi, I've been thinking about upgrading my rig but I'm not sure if I should wait for the 700 series or buy something in the 600 series like a 660 or a 660 ti.

I've had my 6870 for a year now and its done me well, I could sell this card (while its still worth something) and get some money to put towards a new card but I don't know if its worth it and if I should wait, what do you think?

Current Rig:
i5 2500k @4.3ghz
MSI Z68A-GD55
Exceleram 8GB @1600mhz
Corsair 650w
XFX 6870 Duel Fan

Also if you think I should go in for the 600 series, should I get the 660 (£190) or the 660 ti (£250).

Thanks.