I do not know how you managed 5Ghz on a i5 2500k, but yeah, that would work really well. But I think, correct me if I am wrong, but 1155 boards only go 16x or 8x 8x SLi/CF. That would mean that you would take advantage of all that power.
Ummm... Wrong. Anything that is higher than a hd5870 loses performance at 8x. If you cards are stronger than a hd5870 it will lose performance. The higher above a hd5870, the more performance you will end up losing. So in reality, you might end up losing 10 FPS.
Using a board with an NF 200 chip adds latency you know so will it's actually a slight determent to LGA 1366 boards it's a benefit for the 1156 and 1155 boards. So, for a sandy bridge chip, a board with the NF 200 would allow two 4870X2s or two 295s to communicate with each other with 16x/16x lanes rather than just 8x/8x. For four 4890s though you would need a a board with 4 8x slots with an empty slot between them which also means a bigger case with at least room for 8 expansion slots rather than 7 so the last card has an exhaust.
Ebay your dated dual GPU cards if you want money--dump them on classifieds here if you don't.
Then buy a new dual GPU (GTX 590/6990) when it comes out. The 8x can limit your high end cards if you crossfire/SLI them, but you'll be getting over 60fps in anything, so it won't matter. It should matter with decoding though--which should be done with Radeons btw.