Made my proc use all 8 cores but bumped it down to 80%, restarted both (paused and start) and im up to 25k ppd now.
without going back to check (just being lazy) - I assume that's an improvement over your previous configuration? (proc at 100%)

Option 1) OC the existing rig, take the 220W hit and get how much % as PPD improvement? probably less than 20%
Option 2) Keep your system at stock frequencies. Get an additional 660 Ti which will take about 120W when folding and gives you an almost 50% PPD increase vs. your GTX 690.
that's an honest assessment, however I could not speak (yet) to its accuracy. currently, I am running with only the GPUs OCed and the 3930 at stock speeds (I'm waiting to OC until I can install the indigo xtreme - maybe in the next few days). with this configuration I'm only running at +70W since the 690 is fairly efficient even at OC speeds/volts. as such I was able to hit 94k ppd yesterday (!!) (though it still varies quite a bit since I'm home to use the computer, and likely due to different F@H cores as well) - note: previously (without running the 690 at all for folding) I hit a max of 36.8k ppd avg for the month of august with over 1.1 million for the month. it would be too hard right now (running for a week straight without using the PC) to determine the effect of OCing vs. ppd. once I go back to work (in a couple weeks) I can set the OC or stock and then let it run. I will do it for both GPU and CPU (assuming I've installed the IX) to see what difference the OC makes and if it is worth it vs. the power consumption as a cost per ppd comparison. I know the CPU watt/ppd will not be good. sandy bridge - E takes a lot to OC (75% more power vs 33% more freq). the GPU on the other hand, just may be quite good....

as for substituting a 660Ti for the OC of a 690: remember, the OC on the 690 only costs 70W vs. the 120W of the 660 and that you would still have to overcome the 300$ cost (with a quick Newegg search) of the 660. and, I'm not certain the 660 would fair as well vs the OC as you might think. like I said before, I'll run some numbers once I go back to work in a couple weeks.