Now I know like only 60 games support PhysX and that the 670 is capable of pulling off physX and continue on killing games, but say that I wanted to get a different card for the use of PhysX. What card would best work along a GTX 670?
Gotta look at the cards with higher CUDA core counts. Pick your budget, then get the card with the most CUDA cores within that budget. If it were me, I would look at the GTX 460, even one of the cut down, cheap models with less RAM and less bandwidth, like the SE version or the 768 MB version.
By the way, I think it's great that you qualified your question. Without a doubt when someone asks about a PhysX card, there's always people coming on and giving their opinions as to why having dedicated PhysX is a bad idea. If you do it properly and use it in demanding PhysX games like Batman, you'll see a nice impovement, maybe as much as 25% better framerates.
If you want to do it on the cheap then a cheap 8800/9800gt will do the job a lot better than any Fermi or Kepler card that is below the gts 450 in terms of raw power.
I have tested through out the years what makes a Good dedicated card and found that the G92 is more than enough for most games. Resolution doesn't matter as physx is limited to numbers of objects and particles in motion. It can be very easily overclocked and the core/shader clocks are not locked into a fixed ratio.
The next best thing in line is a 9800gtx+ and the gts250.
A gts450 will certainly get the job done but there are older and cheaper cards that will do just fine for a good two years or so. The gtx 670 isn't so powerful that a dedicated card wouldn't make a difference at all when I have seen people bench test a wimpy GT240 as a dedicated card with two gtx580 and still maintain better average fps than just three straight gtx580 in tri sli in physx titles.
8800/9800gt if you don't feel like spending the money and are single slot.
9800gtx/gts250 is you want something with a bit more power plus overclocking to 1.9ghz+ on the shader is nothing to laugh at.
gts450/gtx550ti if you want something modern but still want to hit 1ghz on the core without having a lot of power consumption.
If you want to get something that is overkill then a gtx 280, gtx285, or a gtx460 256bit will provide for many years in dedicated physx.