I dont' know the truth to it, but I think I read that a good indication of which is the bottleneck is. If you're getting the same or close FPS at different resolutions, than chances are its your processor...but if you're getting better FPS at lower resolutions, then its your Vid Card.
Its obviously not 100% true, but it made a little sense in my head.
i.e.
8x6 = 56FPS
10x12 = 56FPS
then your processor can't give your video card enough data to put out more than 56FPS...
but if its
8x6 = 56FPS
10x12 = 30FPS
then your processor is giving your video card enough to do 56 but its only capable of 30 at that res.
I could be making this all up, but I think I read it a few years ago on Anand...it makes sense, and obviously there are 100 other factors to take into consideration, but it could be a good guideline.