Something worth thinking about: a GTX 650 is very similar in graphics performance to a GTX 280 and uses only 64W instead of 236W (meaning a LOT less heat and noise); but the GTX 280 has 240 CUDA cores at 602MHz, while the 650 has 384 CUDA cores at 1058Mhz.
Most graphics cards get a performance bump from a physx card so long as the physx card has over 96 cores, so using a GPU with 240 cores will surely give you a bump in performance. But one thing to consider is that if you are getting a performance bump due to a physx card, but the extra heat produced makes you have to lower an OC that you may have on your 690 you may overall have a loss in performance.
The 650 option evilsofa mentioned is a good one because of the low heat throughput, but increased processing power resulting in better physx performance and lower temps.
One thing I should mention though is that a 280 will be way overkill for most any physx to begin with. I run a GTX260 216 for physx and even with modded profiles for physx in BL2 I never get more than 30% utilization.