I saw your post and thought I'd add to this thread 5 years later. 😀
I've recently got a cheap X51 R2 with an i7-4790 and I upgraded the GPU to an ASUS GTX 1070 Turbo and it can max out Battlefield 1, PUBG and every other game I play, I'm using the 330W PSU which while playing never really goes above 120W usage at the wall, GPU temp is usually around 55-60°C. Great little system the Alienware X51 R2.