Well it comes with 220w PSU, which doesn't leave you much room to upgrade. Also it's small form factor so you have to get a SFF GPU.
VisionTek 900549 Radeon HD 7750 1GB GDDR5http://www.newegg.com/Product/Product.aspx?Item=N82E16814129263
That will work, might cut it close. You should consider a PSU upgrade to get something more powerful.