I think this really only applies to machines that are not dedicated folders.
My rig, which is almost exactly the same as yours - Asus P8P-67 Pro i7-2600k @ 4.5 and cas9 ram@ 1600Mhz (8gig) produces very different numbers
CPU only will average 28k ppd
CPU + 460 stock (GPU3 units) 32k ppd
CPU + 560 ti @ 880 (adv-methods gpu3) 40-42kppd
all 3 together average 46k
I have found running adv-methods GPU3 units results in quite an increase in SMP ppd (presumably because they require less frequent CPU cycles)
Also depends on how many 101xx units you get on your SMP. those units run anywhere from 8-12k ppd less than other SMP units.
I have come to the conclusion that the only way to justify folding SMP and GPU simultaneously is to have them running 24 hours. Otherwise you can get basically the same ppd from running SMP alone because it will run faster and get more bonus points.
I guess each rig has its own peculiarities and usage, so you have to work out what nets the best ppd on your system. As Xanderf has pointed out, it can/does take long runs of many days, and combinations of hardware usage and settings before you find your sweet spot.
It's a bit like old-school FSB over-clocking. You can easily spend weeks trying to squeeze out that last hundred ppd!
(it's almost as addicting too. I currently running a long set of combined SMP + 2 GPU, fiddling with the gpu clocks to try and find the balance between higher gpu clocks/points and smp points, makes for a BIG excel chart