So i bought my X51 R2 awhile ago and i currently don't have the money to by a custom PC. A friend of mine sold me a practically brand new Skylake I7 6700k 4.0 Ghz processor and was wondering if it would fit in my Alienware X51 R2. Thanks!
Looks like your motherboard is socket 1150 and the 6700k is socket 1151 so no it won't work unfortunately.
But he can buy a 4-series i7 like a i7 4970 or i7 4970K (you probably don't need the K-part with a board like that that probably doesn't support overclocking) which is nearly as fast as a 6700K.