gpu replacement?

I suggest you GTX 660 or R9 270 it is good for that money. All the best!

Since you didn't mention power supply, I'm guessing you didn't know it was relevant and that leads me to believe that your computer is a prebuilt model. Most prebuilt models won't have a more powerful power supply than 300. A couple of your other respondents have mentioned the 750 ti. I concur as it is the best you can get without putting in a new power supply.

If you do have more power the best you can get for about 200 is the Radeon R9 270X. If you can find it on sale, you might be able to get it and a better power supply and still stay within your budget.

nvidia geforce gt 750ti

ati rage 128 is "all the rage"

The GF108? That's an Nvidia chipset from 4-5 years ago.

I'm a bit reluctant to tell you that you should get something for your system without knowing more of the parts, but a 750 or 750 Ti would fit in that price range.