Power Supply?

Hi FissionCrayfish‌,

This is the list of validated CPUs for your system:

Alienware X51 R3
Processor Type
Intel® Core™ i5- 6400 Processor
- Skylake-S (6MB Cache, 2.7 GHz)
Intel® Core™ i7- 6700 Processor
- Skylake-S (8MB Cache, 3.4 GHz)
Intel® Core™ i7- 6700K Processor
- Skylake-S (8MB Cache, 4.0 GHz)

Post-RTS
Intel® Core™ i5-6600K Processort Skylake-S (6MB Cache, 3.9 GHz)

So yes, that processor will be compatible with your system.

The power supply you need depends on the CPU and VC. So, let us know what video card you have.

"Video Card?" would have been my second Post and "Yes" my third Post.