Right now, I'm sorry to say but you can't. GPU prices are really messed up at this moment, and chances of them settling back to normal anytime soon are pretty slim. You'll just have to continue to use the current GPU until everything settles down, which would take around 2 months, give or take a week. Once they do settle, you could buy an RX 580 8 GB card or a GTX 1060 6 GB card for that price, or RX Vega if it actually will be priced around that. Neither of these three options are available right now, the first two due to the mining, and the third because it hasn't launched yet. My recommendation would be the RX 580 once it comes back to the normal prices, it's a great 1080p gaming card, and probably won't be bottlenecked by that CPU...