As Mike says they are all basically the same card.
ATI release a reference design to its 3rd party manufacturers so they will all be the same bar heatsinks, shrouds, colour of pcb and packaging etc.
I've used Sapphire in the past and they have been reliable.
BM08 also has a point, the 5670 is quite a low performing card as things stand now.
It will obviously be better than your nvidia 8600, however it wont cut it with the newer games out now.
If your not a gamer then this wont matter, it will perform fine for day to day use.