The real answer is:
It's probably best to run 1 task per GPU.

If you have 2GB or more of memory on the GPU, you can attempt multiple tasks, and if you're not doing other things with the GPU (like suspending/resuming, gaming every so often, etc.), then it'll probably work fine. But if you plan on using the system for anything besides BOINC, then I'd recommend 1 task per GPU.