Hello Glen! welcome to teamLFN!
The most productive client to use on CPUs is the SMP client. See the f@h High Performance Clients page
for more info
You can indeed have separate CPU and GPU clients - as the GPU client does not draw on the CPU much. The only issues here are heat, and usability of your pc. In alot of instances, its quite feasible to have your GPU client running and still do most normal PC tasks, but if the CPU clients are running, this is diminished, particularly if you have all cores chipping in.
From a work perspective, the clients which get the most done are in this order:
GPU nv>GPU ati>PS3>CPU(SMP)>CPU std
I dont have that kit specifically, but I'd imagine your 250GT would probably match all for cores of your i7 running at full pelt - and leave you with a usable PC for non gaming activities.
If it is a HTPC, you generally don't want anything running if that rig is recording TV. You can set windows task scheduler to run the f@h client at certain times when you know it wont be doing any recording.