I have a Dell XPS 720, with a q6600 2.4 ghz, 8 gigs of ram and a Radeon HD6850 card, 750w psu with windows 8.1. What's the best compatible gpu I can put in this the way it stands? I'm only get 20-30 FPS playing Arma 3, DayZ & Planetside 2 on low to medium settings.