Hah, it's upside down. Well, the motherboard that is. You may be able to get away with using a 5670, but with a wimpy PSU you risk blowing out components if you actually manage to put this under heavy load. Then again it's not a monster machine so it could be just fine.
If you are going to get a new PSU then you may as well get a more powerful Radeon 4850 or nvidia GTS 250. The main reason to get a 5670 is since it's low power and heat after all
. Even though the 4850 and GTS 250 are not DX11 cards, they are faster than the 5670 and the 5670 is just a little too slow to use DX11 anyway.