Technically any video card would be compatible as all graphics cards use a pci-e port. I would recommend some form of Geforce 560ti, On ebay you can pick one up for around £60/ $100 or if you hold out maybe a 570 (That does need a 550 PSU). Soon you may be able to get a 650 up to the 660 they are more easy on the psu and don't require such beastly psus) (DO NOT GET A GT CARD FOR GAMING OR GRAPHICS DESIGN) Or from an Amd stand port maybe an R7 250/250x/260. Or the older 7750/7770/7790. For the implied budget. What is the actual budget and what will your usage be? And do you know what PSU it is? (As that will be a limiting factor of your PC). Please post model number and full specs. Also some dimensions of free space in the case and how many pci-e conectors would be useful. If you aren't doing graphically intensive tasks you could get an nvidia GT 600/700 series as they will handle most things Okay and have low power consumption, but otherwise don't. If you play games what do you play this can sway your choice as different games prefer different architectures and manuafacturer.
If you have a larger psu check out the nvidia 400 series gtx460-gtx490 they are cheaper and still fast.
To see if your PSU will take a GPU use this chart (Its helped me many times)http://www.realhardtechx.com/index_archivos/Page362.htm