The system includes an integrated GeForce by default, and has one PCI Express slot, but it's low profile, so that'll cut into upgrade options. The system also only ships with a 220 watt power supply, so that also limits options, and I'm not happy with the state of upgrades for power supplies in this formfactor, so my advice is to stick with something in a low enough power envelope to make it work.
Unfortunately that severely limits the options out there, and the best thing I can find from a performance standpoint is one of these cards:
Newegg.com - SAPPHIRE 100291DDR3L Radeon HD 5450 512MB 64-bit DDR3 PCI Express 2.1 x16 HDCP Ready Low Profile Ready Video Card
The photos shows a full size bracket, but we buy these for the office and they come with the low profile bracket to change it into the form necessary to work in his case.