Since its a prebuilt you'll need to check the power supply before we can be sure the next range of cards are a good idea. What is your current PSU?
If the PSU is sufficient, I would recommend the GTX 970 for your rig. At 1080p you will max out every game on Ultra, and have a more effcient card than the R9 390. For a pre-build running the FX-8320 the GTX 970 would be a great choice, as long as your PSU can handle it.
Video Card: MSI GeForce GTX 970 4GB Twin Frozr V Video Card ($328.99 @ SuperBiiz)
Total: $328.99
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-12-02 08:41 EST-0500
*Required power: 1x 6pin PCI-E Power Connector, 1x 8pin PCI-E Power Connector...