PPD is a bit more than an e-penis. Its a approximate value of scientific work. The next psrt of ppd is the return speed.
In folding, the next wu is not ready in advance, each unit builds off the last, so the faster they get back the info the more useful it is to them.
GPUs process very quick, but kind of small an not complicated work. 1350pts/wu 0 return bonus, takes about 2-3hrs on mid level equipment (gtx560ti)
CPU a4 and a3 units calculate larger and more complicated work. 552pt/wu 3hrs to do, bumping it to 4600pts/wu for 22k ppd (i7 @4.0ghz)
Now bigadv wu are huge, very complicated wu, in molecules and trajectorys, 22706pts/wu takes my system 3 days for 250k pts or 78k ppd.
Each have very specific uses to f@h, and each rely on each other. i.e.
gpu's do the quick n dirty work, can be reletively inexpensive to purchase, but add heat and increased power to a system.
cpus do the lets see how things work work. Cost isn't bad, but means you might have more upgrades... more power freindly.
bigadv units require server gear, they do very specific work that take alot of cpu power. High priced, but are fairly power efficient, nd when calculating watts/ppd its the optimum.
As you see everything is needed, and some equipment currently just doing one type of illness.
So as i rambled, doing anything is helpful, being able to spend money to do more is fantastic, but f@h is still Folding At Home, so doing what you can is the most important thing.