i try 4 ssd ocz core raid 0 and maxi was around 600 MB/s reading
Is it worth upgrading from a Q9550 to a i7 920?

Easy to prove; go to Darlen and look at your HDD light. Light off in someplace where there are no people (look at wall) FPS is high. Turn and look at people HHD light starts blinking big time and FPS is low. See for yourself.

Lax also thinks you could be cpu limited. My chips are running at 3.2GHZ, I havent looked at assembly code in awhile but what 8 clocks at most for a multiply? I can't see what wow is doing that you need like millions on million of multiplications or whatever each second. If its cpu limited its cause the dma part of the cpu is moving data from your hard drive to system ram to cpu to video ram.