The Dell Inspiron 531s limits you to low profile graphics cards that don't require a PCI Express supplementary power connector and can run with your 250 Watt power supply's +12 Volt continuous current rating of 17 Amps.
Look for low profile versions of the following:
Radeon HD 6670
Radeon HD 6570
Radeon HD 5670
Radeon HD 5570
What is your budget? Are you willing to replace your TFX power supply unit, if necessary, to be able to run a more powerful graphics card than the one's I've listed above?