How about the 6670?
I've has experience upgrading PC's pretty much the same as yours. I managed to fit a GTX 465 in one and a 6870 in the other, upgraded the PSU in both in order to power those cards. The one i linked to above is quite short so it will fit and consumes 66W which means it can be powered by the PCIe slot on your motherboard. This means you won't have to upgrade your PSU.