There seems to be a bit of misleading information here. First off the latest Nvidia driver(258.96) still supports Physx and setting a dedicated physics card. In the games that support this it will increase the performance of the game(higher FPS) in games that don't there obviously won't be a performance gain. SLI(and the sli bridge) is not required for Physx, it even states in AE_JB's 2nd link that in a multi card setup(not an sli setup) gpu 2 does not have to be identical to gpu 1. In my rig I run an GTX 295 that's dedicated to graphics and a GTS 250 that's dedicated to Physx. With a 1440x900 monitor your current setup should be good for a while but, if you're looking to get a bit more video performance for the newer games then I would get the GTX 460 and use it has the graphics card and use your 9800gtx as the physx card, your current setup should have no problems with this upgrade.
