I have six XP clients running right now - four GPU and two SMP - spread across five platforms. I originally had an additional CPU client running on one of the GPU platforms but killed it after checking the numbers. It turned out that running a single core client on a E2200 knocked about 1400ppd off of the GPU client (8800GT) that was running on that machine (from 5100ppd down to 3700ppd). Even though there was one core of the E2200 completely free to support the GPU, the GPU client was bogging down. Maybe the lowly cache on the E2x processors has something to do with it. /shrug
On the other hand, I have an Opteron 165 running an SMP client (4 cores running on a dual-core processor) AND supporting a GPU2 client (8600GT) and the GPU client only took about a 200ppd hit when I loaded the SMP. Perhaps the smaller hit is due to the lower demands placed by an 8600GT versus an 8800GT. Again, hard to say. I CAN say that there was very little difference in the GPU ppd when I changed the Opteron from the x86 client to the SMP client.
I guess the message is that if you're not sure if you can run an additional CPU client, just give it a try. But it looks to me like the more mojo you have in your GPU the more likely you are to take a ppd hit from adding a CPU client.
This problem was caused by Windows, which was created by Microsoft Corporation.