Hm, if you're only getting 8.5 Mhash my guess is that the GPU mining isn't kicking in and it's just using your CPU. Which miner are you using?
I get 65-70 Mhash on my overclocked 460.
That's weird, because I was using a CPU miner at the same time, which was also pulling about 9Mh, and on task manager it showed it was using all the CPU, while the CUDA miner was using 0.
I'm using GUIminer, with puddinpop's CUDA miner.