lol you wont lose any performance in 8x/8x dont worry.
Unless you have a highly OC/ed GTX295 or 5970 it's not gonna be a problem.
The slots are in "8x/8x" but the second slot just steals the first's bandwidth so to speak. If your RAID controller card doesnt suck up all the bandwidth you should be fine. (You would be fine anyway) :yepp:
Also, 4x would bottleneck the RAID card depending on how fast/how many the drives are etc.

