http://www.overclock.net/t/1244279/folding-with-i5-2500k-and-radeon-hd-6950-ppd-help
In my experience, using two CUDA GPUs requires a "dedicated" CPU core. It looks like taking a 2500k (@ 4.5Ghz) from 4 to 3 cores cost ~6k-10k ppd (this is skewed due to bonus for completing them faster, GPU has no bonus).
To me, this means that 7k ppd is what you are going to get out of a 2500 running two GPUs. Your options look to be:
Use -smp 3 and earn 7k ppd on your CPU, perhaps your GPU PPD will go up because right now they are probably having to wait from time to time for you CPU, running -smp 3 will give them a dedicated core and alleviate that potential bottleneck
OR
Drop the GTS 450 and hope that your CPU can make up the ground with -smp 4 (this should use less power overall)
So you are at ~33k ppd right now? You could likely do that (on a good day) without the GTS 450 so long as only one card allows your CPU to work more efficiently.
This is one of the things that makes the i7's nice for folding, you can free up a "Core" that is really just HT sharing the resources of a core, in this case you'd like go from -smp 8 to -smp 6. That would have less impact on your CPU score because all of the critical CPU resources are still available to CPU slot (FPUs, etc.) With an i5, you have to give up an entire actual core, and that is more painful.
The upshot to GPU folding is how reliable it is. The CPU work, not so much.
Perfect solution? Find another box for the GTS 450 to fold in