i5 750 and 5870 is the best option. That rig will last you a few years easily, more if overclocking. By then whether you bought i7 or i5 socket will be irrelevant as the new stuff will use neither.
As for XFire, just buy a decent P55 board, x8/x8 instead of x16/x16 is actually very little performance difference, you wont even need to XFire the 5870 for a long while and when you do the performance difference will be so small you wont even notice it.
The performance of an i5 750 and 5870 will be notably better than an i7 and 5850, and that will be a bigger difference than the difference between 2 x 5870 x16/x16 than 2 x 5870 x 8/x8, so worth getting the i5. Also, if you spend more on CPU and get the lesser card when you XFire you will go with 2x 5850, whereas if you get the i5 750 your second card will be another one. 2x 5870 x8/x8 will run better than 2 x 5850 x16/x16