I have done a few test with my system today. I have been running a single WU on the GPU.

i7-860 @ 4.0 GHz no HT, ATI 5850 @ 900/1000 MHz.
Only GPU work, no CPU work.
At the start of a WU it runs 13 seconds on the CPU. After that it uses on average 51 seconds during and after GPU use. Average GPU time = 120 seconds.
The picture shows Total time and (CPU time)
HCC_GPU_4GHz_noHT.PNG

i7-860 @ 3.3 GHz HT, ATI 5850 @ 900/1000 MHz.
Only GPU work, no CPU work.
At the start of a WU it runs 19 seconds on the CPU. After that it uses on average 58 seconds during and after GPU use. Average GPU time = 120 seconds.
HCC_GPU_3.3GHz_HT.PNG

i7-860 @ 3.3 GHz HT, ATI 5850 @ 900/1000 MHz.
GPU and CPU work.
At the start of a WU it runs 25 seconds on the CPU. After that it uses on average 87 seconds during and after GPU use. Average GPU time = 131 seconds.
HCC_GPU_CPU_3.3GHz_HT.PNG

i7-860 @ 3.3 GHz no HT, ATI 5850 @ 900/1000 MHz.
GPU and CPU work.
At the start of a WU it runs 16 seconds on the CPU. After that it uses on average 62 seconds during and after GPU use. Average GPU time = 128 seconds.
HCC_GPU_CPU_3.3GHz_noHT.PNG