Tab left for RAM, right for storage.

https://www.crucial.com/compatible-upgrade-for/alienware/area-51-r2-(late-2014)

This isn't acurate.....It thinks there is two ram slots.

The desktop has 4 ram slots.... though there seams to be some confusion on rather it's a 32 or 64 ram limit.

The hardrive is set up for 64 but there seams to be some software limitations i can't find if they are fixed or not yet.

Any 2.5" or 3.5" sata desktop SSD or HDD will work.

If over 1TB, then the boot / boot drive needs to be uefi / gpt.

Also, the Dell service manual lists it as validated to 64GB RAM.

As an aside, the Area 51 R2 was sold stock with a 64 GB ram upgrade option. When it first came out 8GB sticks were the max but 16GB sticks bumped it up to 64GB RAM.  There are no hardware limitations on supporting 64 gb.

"This isn't acurate.....It thinks there is two ram slots." 

The link says there are 4 slots.  2 banks of 2 means two channels, 4 slots.

https://www.crucial.com/compatible-upgrade-for/alienware/area-51-r2-(late-2014)

What about 64gb ram compatablity? Can it do it?

And it don't show what mechanical hard drives are compatable or dose that really matter?

So this should work?

https://www.amazon.com/Mushkin-SILVERLINE-PC4-17000-Dual-Channel-MSL4U213FF16GX2/dp/B07CGNH7JS/ref=sr_1_2?dchild=1&keywords=DDR4-2133+PC4-17000+288-pin&qid=1625527509&refinements=p_n_feature_twenty_browse-bin%3A16158157011%2Cp_n_feature_four_browse-bin%3A10656894011%2Cp_n_feature_five_browse-bin%3A9559990011&rnid=673240011&s=pc&sr=1-2

And can i just buy two and buy two more later.... or do all four slots have to be filled?

I know thats diffrent on some mother boards and stuff and it's changed......  One you could have 1,2,3, or 4 sticks in long as they where all the same..... but another you had to have all 4 filled.

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https://www.amazon.com/Crucial-DDR4-Desktop-Memory-CT8G4DFRA266/dp/B08C4QF5N4/

Alienware Area 51 R2

X99 MS-7862