look at the ebay link, and look at the motherboard.
see the red/orange slot? thats where you put the graphics card, but when you get them, make sure you know how to put it in, you can either attempt it yourself like i did, follow the instructions, or ask people online/watch a video (if there is one).
and yes, that computer can run Microsoft Windows XP, even Vista, but not that well in vista
Desktop -Cooler Master NV690/AMD Phenom II X4 965 BE/MSI 880G-E45 Motherboard/2 x 2GB DDR3 G.Skill Ripjaws X/Sapphire HD 5870 1GB GDDR5/CORSAIR CMPSU-550VX/Seagate 7200.12 500GB (OS), Samsung F3 1TB (Storage)/Sony DVD Burner/Windows 7 Professional 64-bit