Your proc definitely doesn't bottleneck your system. You have you E6550 at a nice OC and it is also a relatively recent dual core... I wouldn't worry it at all. But why not just get a HD4870X2? The fact that it only requires one pci-e lane kinda beats out the buying a crossfire set-up (especially on an 790i). Or you can buy another gtx285 for sli if you can find a cheap one lying around (just make sure your psu is hefty enough for the job).
edit: what is your screen resolution though? if its 1920 x 1200 or lower, then I would just stick with your gtx285 as there would be no notable gains if you decide to purchase 2 hd4870s, 2 gtx285s, or a hd4870hd.