Yes but the rest of the system can't be crap or the overall fps will be lower than the 670 on its own. The gt630 is fast enough for dedicated physx on its own and is the bottom end that one can go without being too slow. If you want to go full settings then I suggest a more powerful card such as a dirt cheap 8800/9800gt or a gts450 if one can be had for cheap. The rest of the system needs to be very decent or it won't keep up with both cards.
If you are only using 1 monitor, then I'm not sure you need a physx card at all. Still, if you really want one, I would go with what nforce said and get something like a 9800gt as it is still more powerful than a gt 630. Basically, if your physx card is too much slower than your regular gpu, it will just slow down everything. The gtx 670 is already a powerful card to begin with.