Most current GPU processing actually only needs very little of your CPU and typically will run at a higher priority when it does need your CPU than WCG tasks so it will get served first. I have sucessfully run 8 WCG WUs with 2 GPUGrid WUs simulateously on my i7 / GTX295 with only a minor decrease in WCG production and no difference on GPUGrid production.

If you still want to do that, you can go to the advanced BOINC view, preferences, processor usage.

On your four core system,
change the number near the bottom to read

On multiprocessor systems, use at most [ 3 ] processors.

That will reserve a processor for other tasks, or leave it idle. There will be no need to suspend a task sitting in one core. BOINC will take 3 cores automatically.

p.s. for others: The number of processors defaults to 4. For those with an Intel i7 (four cores with hyperthreading), they act as 8 cores, and this number should be changed to 8 for full use.

The old hyperthreading on one core systems was quite inefficient. Most of the time we recommended that it be turned off in the BIOS.

The new one won't give the equivalent of 8 physical cores, of course, but is enough of a gain to make it worthwhile for pushing out workunits.

Yes, in version 6 clients it is this percentage parameter which matters.
Actually setting it to 99 % is enough to forbid usage of one thread. Doing this way you can use the same profile for any machine, 2-core, 4-core and 4-core HT or more without having to worry about the total number of tasks that the device can run.

Cheers. Jean.

PS: retsof's considerations about P4 HT are a little outdated too. Many members run them on two simultaneous tasks without problems and with a global throughput about 30 % higher than in single-task mode.
And in case of usability problems I would recommend to tell BOINC to run one task only rather than disabling the HT at BIOS level, precisely because HT helps to smooth the coexistence of multiple processes in the machine.

I did say above that the OLD HT had problems. Did they add some floating point registers, I wonder? It used to thrash trying to get to one.

I also said that the NEW HT was better.

[retsof: I did say above that the OLD HT had problems. ... I also said that the NEW HT was better.]
Something that is not prominently stated is that while Windows 2000 was/is HT-aware and runs processes on both P4 virtual cores, it does not use them efficiently. Intel or MS - not sure which - recommend that HT be disabled under Windows 2000. This problem was fixed in XP. Perhaps this is what caused the discrepancy in findings between the "old" and "new" HT on P4s.

Win2k can also recognise only 2 CPU cores maximum, virtual or real, so running it is not an issue with HT on the new and soon-to-arrive 2, 4 and 6-core CPUs.

BUT WE ARE OFF-TOPIC:
skgiven's original problem was how to get one BOINC project (GPUGRID) to use 1 core 24/7 and another BOINC project (WCG) to use the other 3 cores.
No-one has answered this. Perhaps it is not possible without recompiling your own version of the BOINC client.
Or perhaps it is possible to install and run 2 instances of BOINC, setting processor usage to 25%/1 for GPUGRID and 75%/3 for WCG.
(This had to be done with the old WCG UD client in order to run multiple CPU cores).
Is it possible to run 2 operating systems with actively running tasks simultaneously on the one machine?