Boy, how did I guess ... another Dell or HP slimline question.
Your best option would seriously be to take everything out of that case, put it in a regular-sized case for about $30, and then get a decent power supply of like 500W+. Then you could put whatever video card you wanted in it. As it stands, you have a 275W power supply, which I wouldn't trust with anything better than a 4650 regardless.
Make sure to avoid slim cases at all costs in the future -- they usually come with a terrible PSU which limits what you can do in addition to space issues being a problem. Basically, it's like getting punched in the nuts twice in a row. They basically make slimline systems so you can't do any worthwhile upgrades; they want you to buy a whole new system instead.