Crossfire In x16 and x4??

Assuming that no other components have issues, it should work pretty fine. The 7770 shouldn't really be bottle-necked by a PCIe 2.0 x4 slot and Crossfire does work that way.

Due to the PCIe slots I don't think you'll have much of an issue. Your processor may cause a bottleneck with crossfire though.

Difference between PCI-e 2 x16 and PCI-e 2 x8/x4 is so minimal, that it's not worth worrying about this AT ALL.
Difference between x16 and x1 can be a problem, but not if you do "x16 + x8" or "x16 + x4" combo.
It will be slower, but not so slower that you could actually see a concrete difference.
It usually matters only to FPS-addicts who count every frame, not the absolute majority of mainstream users.
99.99% of mainstream users wouldn't be able to see any difference AT ALL.

Thank you for the reply's, sound's like I should go for it.

Cheers!

Would the corsair builder series cx500 be a decent power supply for a pair of 7770s? if not then what would be that's great on a budget?

CX500 is fine as long as you don't happen to have a pair of Xeons or quad opterons for the rest of your build(as long as the rest of your build is normal).

A 7770 will run fine on x4/x16