I hate to ninja the thread but in a few weeks I am looking to get a low end card for dedicated physx. It needs to be single slot and the cost has to be as low as reasonably possible. Reason is due to the increased demands of physx is having a small but noticeable impact on the performance of my gtx 460.
As for my input the GT240 is better but both are two weak unless you have no choice but to stick with your limited price range. The GDDR3 version of the GT220 is the one to look at while the GT240 typically is equipped with GDDR5 and any other version should be avoided. Bother are horribly crippled due to their poor fillrates and low bandwidth. Expect 800x600 with the GT220.