Graphic Card for Gaming, and also depending on which graphic card u get u might have to get a better PSU. Also $300 isnt too bad for a graphics card.
can u list all of ur components that u have currently
I'm liking M0j0jojo's build better than mine, I really like how he got a GTX 660 in there. I prefer Nvidia when possible, seems they draw a lot less power so won't be as hard on your power supply.