How this setup will perform on a ep45-ud3p gigabyte mobo? Will the performance be the same compare to a full pledge xfire board or it will experience some bottleneck due to pci-e slots running on x8? My other concern is my PSU, i have a corsair tx 750w will this thing be enough to run two 4890 on xfire?
There really won't be a HUGE difference in x16 and x8. It's not like you're going to get half the results you would if you ran at x8. You could wait for the 4890 x2 but it'll be damned expensive. You'll get better performance though for sure. Even if both slots run at x16 you won't get the same performance out of 2 4890's as you would if you ran one 4870 x2.