AMD 6770 $99 (100% value = 80fps.)http://www.newegg.com/Product/Product.aspx?Item=N82E16814102969
AMD 7750 $110 (105% value = 85 fps.)http://www.newegg.com/Product/Product.aspx?Item=N82E16814125415
AMD 7770 $120 (130% value = 110fps.)
Simply put, don't hamstring yourself for future gaming, spend an extra 20 bucks, stop with the takeaway latte's for a week or three and enjoy your rig for years, rather than months.
It's compatable with your 550W PSU no matter the brand as the new technology makes it rather efficient.
It will game up to HD, but it's sure as hell not going to be bottlenecked by your Q6600!
So TL:DR don't be a stinge, spend a wee bit extra, and in that price bracket the performance (read: gaming life) by far beats the price difference!