Dell Optiplex 990

Wound up with one of these in working order with Win11 on it:

https://ibb.co/0ybHT8qV

I see one stick of RAM in there, I guess its an 8GB stick?
HDD is the stock 500mB drive.

I noticed the CPU has copper pipes going to it for cooling....
I am wondering if its worth the time to take the fan off and inspect the 
thermal
paste and replace it?

Apparently W11 is not well suited for this hardware, but I am no fan of 10 
either.

Considering a Linux flavor such as Mint or Cinnamon, or are there any other 
interesting OS's
that would kick butt on this one?

Not really looking for gaming on it, but the video chips seem to be half way 
decent for that.
It will be mostly a small business desktop use, making webpages, office 
documents, does not have to be a screamer
but I guess there is another memory slot if need be and this is probably the 
fastest CPU that it can have?