When memory of different speeds is installed, memory speed and timings will be set to what ever the slowest memory can run at.
If you already have 2 gig of memory, installing more memory, even more PC3200 is not going to make any noticable difference. Installing slower memory will most likely cause a noticable decrease in performance. Some boards may not like the mixed modules and not even boot. In short, I would not install them.
If on the other hand you only had say 512 meg and were adding another 512 meg, then I would say give it shot. The added memory would be beneficial even though it would be running slower.