Well, with the case you have, your options are limited. You can basically go up to a low-profile Radeon HD7750 on that computer and it would run well.
Also, if you want to go with more, you can upgrade the case, power supply and get a full size video card. The motherboard in the Dell seems to be of standard size, so you can get a new case and mount the motherboard in it.
An upgrade in RAM would also be a good idea.