i7-930. with 24 gb ram, i now have 2 gtx 460's <1gb sli, 750w powersuply, what gpu upgrade is better without blowing my system up? im not willing to spend more than 300$
You can use any PCIe card you want. Your resolution and PSU is going to determine what the best choice is. If you have your CPU OC'ed, you can achieve good FPS with your current system. The Rx 480 or GTX 1060 6GB are most likely your best bets.
Be sure it's properly seated. The monitor is connected to it, and the PSU power connector is connected. Otherwise, I don't know why it wouldn't be recognized. You might check the device manager, and see that there isn't an exclamation mark next to it.