1 high end gpu or 2 medium level gpu's

Hey everyone, jus looking for some good solid advice,and this is BY FAR the best place to go for advice....Here are my specs: Cooler Master HAF 912 case, intel extreme DP55KG mobo, intel i5-760, 2x4 Patriot Sector 5 G series RAM, 1 60G Samsung SSD Boot Drive, 2 Samsung HD502HJ 500G hard drives in raid 0, Samsung Blu-Ray Drive, 3 x Dell 23'' 1080p monitors, Xion 800watt PSU, Cooler Master Hyper 212 plus, Windows 7 64 home premium, and all Cooler Master fans....I understand that my mobo has 1 pci-e x16 and 1 pci-e x8 effectively making them both pci-e x8 and from what i understand it shouldnt be that bad of a drop in performance, all though not ideal, but not an absolute no-go! So my question is...should i sell both of my MSI GeForce GTX 550ti's in SLI for a 570? (if i can) and then hope and pray to try and get another 570 someday? So basically 2 x 550ti's or 1 570ti? (with the possibility of a second card in the distant future) 3 monitor setup is a must, since i have all 3 of the same monitors allready...or is it all kind of a waste with the mobo and components i have to go for that high in the GPU range? Any / all info is / will be GREATLY appreciated! Thank you!