I wold make sure that the power supply is sufficient. Most likely it is a 300W generic power supply, which would most likely be only good enough for a Radeon 5670/6670 (not that that is a bad thing though.
I have one similar to yours, and I replaced the power supply and am running a Nvidia GTS250. The 5670 is probably the highest you could go without a power supply, the 250 is probably the highest before a bottleneck might occur.