Under $200 is very restricting. You'd need a new CPU and motherboard. You could try getting an APU - a good one can let you play modern games at 720p at medium; the top of the range one (Iris) will get you playing normal games at 1080p at medium settings.
If, however, you want a graphics card you could look at getting a $60-ish AMD CPU (intel is out of the question for this budget), a cheap motherboard for $40 and investing $100 or so into a good graphics card.