That computer has a 300W PSU, so that's going to restrict you quite a bit. I don't have any recommendations for a low power GPU, but I would recommend adding some memory to that computer. The spec sheet I saw listed that it has only 1GB of memory. That is going to cripple performance and make the computer extremely unpleasant to use.
Since it's DDR2, you're going to get gouged a bit on the upgrade, but you really want a minimum of 4GB of memory.