You want a Crossfire, i7 Sandy Bridge setup but you want to majorly skimp on the motherboard?

I've dealt with budget boards in the past, don't go there. Get at least a mid-range board. I'd be looking at a UD4/UD5 level in the Gigabyte range personally. That will give you 8x bandwidth on each PCIe slot if you're running CF but unless you're going top-end cards, you probably won't soak up the available bandwidth.
Well I might be able to stretch to one of those, I'm currently selling something on ebay which will fetch me in at least £520 after paypal takes it's chunk so I need motherboard, cpu and RAM for less than that. The second card will come later out of some other money.
Ideally I want to save a bit of cash along the lines but I still want a good stable system.