4890 crossfire

x8 lanes will choke a HD 4890 XFire setup. You're better off with a single HD 4890 since it will get all x16 lanes.

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.

Keep in mind that it's PCI-E Gen 2, which is twice the bandwidth of PCI-E Gen 1, so X8 lanes will be equivalent to 16X PCI-E Gen 1. There would be very minimal to no performance difference. Trust me.