One thing you might wish to research - and I profess no knowledge whatsover about it - is using an external graphics card - probably connected by USB which I presume will have its own power supply or connection to household power. However, with an enclosure and some kind of power provision, it is likely to cost substantially more - and obviously with a USB maybe work more slowly. Just a thought.
If budget allows, you mgiht also consider changing out your PSU, which also might be cheaper than the external video card in total A good PSU for a modest increase (would run good mainstream cards but not enthusiast level video cards) would be something like this:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371006
If that is too step, a midrange choice might be the Coolermaster eXtreme Power Plus 500w for $30 after a $10 mail in rebate.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817171031
You can find PSUs for less, but I would not recommend a low cost one due to the damage it could cause to your computer.
If decide to get a new PSU - you need to make sure it will fit your PC. Dell used to have their own size but I now they use standard components. I don't know when the change occurred. YOu couild measure your current one and make sure the replacement is no larger. Also I would strongly suggest going to the Dell forum and searching to find others with the same model tha thave already upgraded.
You should also take a quick look inside youir case to ensure that you can remove and replace the power unit without dismantling other components such as a heatsink/fan, or at least be aware of what will be required, It actually is quite simple. You just unscrew a couple of screws to release the power supply and pull it out - without detaching the cables. Then replace each old cable connection with a newl Just undo the old connection and insert the new, then move on to the next cable.