Hello, I'm getting an XFX 5850 black edition for free from a friend. currently running evga 9600gt (yes i know its old lol). I can find good deals for another 5850 for less than 100 bucks and wanted to do crossfire with them. would 2 5850's be as fast as getting a new gtx660? or 7870
More aboutfree 5850 upgrade
I would stick with your current 5850. Its a good card.
Also, make sure you have a suitable power supply.
keep your free card. no sense in thinking they will "be as fast as"............. frames per second aren't the only way to gauge a cards performance. the 5850 and the 660 are completely different animals.