I think you could not really pull a GPUminer on bots - that usually need drivers installed correctly etc and it really slows down system performace - I think it would slow down their computers too much so most of the victims would simply buy new hardware.
However, I'm administering a Computer pool with 80 machines here for work, and thos machines have to stay on at night but theres no one being able to use them, so what I can tell:
CPU: Intel Q6600 -> 4 MHash/s/Machine
GPU: GF 8800 GT -> 25 MHash/s/Machine
that makes roughly between 25 and 30 MHashes per machine, for me it's about 2GH/s, but of course a large scale bot network could do much better.
If using poclbm, use the flag -f120 or -f180
It will assign mining a super low priority in comparison to anything else that is requesting GPU resources. I can game perfectly well with it on. Dropd to about 40% of my hashing potential, and when idle i get about 3-5% lower hash.