What card will give me the most performance with my cpu?

Unless you get a very weak GPU, it's usually the CPU that causes the bottlenecking in games ( though how much of a bottleneck depends on the game and your graphic details ) So the simple method is just to buy the best GPU you can reasonably afford. If you have $300 to spend, that means a GTX 970 or R9 290X are about the best you can do right now.

There are other concerns of course. What resolution you're playing games at, whether your power supply can handle the card, whether the GPU can fit in your case, and whether the GPU can keep cool enough in your case.