yeah, just get the 750 ti and plug it in... after you check to make sure you power supply has a free 6 pin power connector..
http://img.dxcdn.com/productimages/sku_28979_2.jpg
Your power supply should have one of those... Then all you need is:
- turn off the computer ( I know.. but my friend plugged in his graphics card with the computer on...)
- unplug the power from the back
- plug in the graphics card and fasten it to the back of the case.
- plug in the graphics card power.
- boot up the computer and go to the nvidia website and download the latest driver...
good to go.... dont forget to close the computer or you might have a dead cat on your hands... lol