While I'm not certain of the effects of running SMP & GPU clients on the same machine it's what I'm doing!
I think ATI GPU client consumes *a lot* (like a core of) more CPU power than the Nvidia one but under Vista at least I found a fix for it.
Unless the system has a weak CPU I think any slight drop in GPU performance should be more than made up for by the SMP PPDs.
I don't think your E7200 will have any problem running both unless you've got massive GPU folding, like 2x Nvidia GTX275/285 or GTX295.
I read a post here by someone with a moderate Nvidia GPU on an Atom system and the conclusion was better to let the Atom CPU only feed the GPU client and not fold itself. This is obviously an extreme case as the Atom CPU is very low power compared to modern Core CPUs
I'm running SMP + GPU on my two PCs, E6600 + GTX260 and Q9400 + GTX260. The E6600 @ 3GHz runs SMP around 1500ppd, the Q9400 @ 3.2GHz runs 2500+ppd and the GTX260s run 7000-9000ppd depending on shader clock and unit. (all according to FAHMon)
Screen shot of my main PC running SMP + GPU folding
Hope this helps, Seb