That PPD is about right for an old-style 88GTS. You could probably get a few more by clocking the shaders using RivaTuner.
Yeah, sounds like you're on the Windows SMP client. 1600ppd is a bit low for a quad, but you might just have a low-scoring WU. What's the project number of the SMP WU you're folding? And how much CPU is being used by FahCore_11.exe?