If upgrading your power supply is out of the question (I would do this first) I would go for the Radeon HD 6670, it is not a great card by any means, but will just barley run with your PSU. The 6670 reqires at least a 400W PSU, but I wouldn't feel extremely comfortable. If I were you I would get a new PSU, around 650W or so (650 will run most any GPU out today.) Then upgrade my GPU to something like a Radeon 6770 or 6850. You could probably do all of this for less than $200 easily.