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.
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.