I too have an Alienware Area 51 R2. The power supply has started to act a bit wonky and sometimes it won't start up properly before I remove and insert the CMOS battery. I've changed to a new battery so I'm fairly sure it has nothing to do with that.

Today I bought a new power supply, Corsair RM850, but I couldn't get it to work with the computer. 

Does anyone here know any power supplies out there that actually works with the Alienware Area 51 R2?

Thank your for the reply. All those power supplies are pretty expensive though. Doesn't newer PSU's work? Like maybe Seasonic Focus GX 750W?

yes a less expensive common psu from EVGA would work.  there is really no need to go after the high end boutique brands.  Seasonic is a good name.

Thanks a lot for the help! I just want a PSU that isn't too loud Ordered the Seasonic Focus GX750 and it will arrive by tomorrow. I'll get back to you here to tell if it works or not

it should work. just make sure use only seasonic modular cables for the GX750 and take out all the Dell psu cables.  and you will need to use two 8 pin eps seasonic cables for the r2 motherboard for cpu and gpu (#30 and #18 respectively).

It still doesn’t work… I mean, the power should at least work so that the PSU starts, but it doesn’t. Also the 6+2 pin PCIe cable doesn’t fit in the PCIe PWR slot (number 18 in the picture). It fits only as 6 pin but nothing if it try to insert the extra 2 pin. 

I really don’t get why the PSU won’t even start though… I tested the PSU before I even started to connect it to my PC and it is operational.

Re: 6+2 pin PCIe cable doesn’t fit in the PCIe PWR slot (number 18 in the picture)

you need to plug a second cpu 8 pin connector (EPS) to the PCIe PWR socket (#18), not 8 pin PCIe.

PCIe

This site should help you identify the correct connectors to use, based on the motherboard connector layout. All these connectors will have round and square connecting pins, to avoid plugging in the wrong connector.

However, with enough force the wrong connector can be forced into it, causing permanent damage to the components pending on what was pushed into it.

PSU Connectors 

What you are looking for is this picture: 8 PIN EPS 

Notice the layout of the board connector in the left picture: Square-round-round-square for the top section,  and round-square-square-round for the bottom section.

If you tried the 6+2 PIC-e cable, that will not work: 6+2 PCIe 

As an almost universal rule, those cables should only go directly to video cards, never to a motherboard. 

I forgot to mention that I also tried the CPU cable and it still didn’t work. Could it havent anything to do with the power cables going to the the hard drives? They’re some special custom kind of cables that’s can’t be switched for new ones. 

Oh yeahhhh... forgot about that little thing. Are you sure clearing CMOS will make it work? I'm too tired to try right now since I already put back the old PSU, so I'll be trying it tomorrow.