The HD 6670 should be fine. Any card that does not require a PCIe power cable will be safe to use. If you have a full size case (no slim-line), you can use any HD 6670 you prefer. Some 300W Inspiron 530s have a PCIe power cable from the PSU tempting you to use a larger graphics card. That's because Dell used a PSU for that model that was closer to a 350W PSU than 300W. I would still stay with the cards that get all its power from the PCIe slot itself (75W) though.