A 5670 is a pretty good choice I think. Its as much as you want to use with that system anyway i would think and a good last upgrade before a new system.
You could even carry it over to a new system eventually until you could afford a newer better card.
Aye, the 5670 is a good low power card that can actually game. The 6670 is the fastest AMD card to not require a power connector, but it is quite a bit more expensive than the 5670 and only a bit faster, so it's not recommended.
Just make sure you avoid a DDR3 5670 as they are slower.