GPU's aren't that good for Monero, i suggest you read some more.
Go to Google, typ in: Mining Monero coin, and you should find enough information.

What did you read that lead you to believe that Monero was the only profitable option to mine? There are many many altcoins that are more profitable for GPU mining then Monero, even with older hardware.

It's what I found searching around the net. SHA, X-11 and Scrypt are being mined in large farms. So, monero is one of the stable coin to mine for profit.

The 290x is a monster, great card, uses a lot of power though.  I get like 740h/s out of it, compared with 650 for my 480's.  Love that card, but it's best if your energy costs are subsidized.

Advantage of linux is
- low hardware overhead
- great for networking
- easy to run headless
- reliable
- free

Advantage of windows
- more tools in windows to optimize GPU performance
- more compatible with some modern motherboards
- less learning curve initially (I guess.  I wasn't eager to maintain a windows network)

Consider old, refurb business or workstation motherboards.  HP makes good ones, just make sure they have standard power connectors.  Old is good for linux, you really don't want to have to deal with UEFI if you don't have to.  

That said, I've had good luck with MSI UEFI MB's.  They tend to come from the factory with the right settings.  Still helpful to add "iommu=soft" to your kernel parameters (google it up).  If you don't, sometimes half your USB ports go dead, which doesn't bode well for your USB drives.  You don't run into those issues with older pre-UEFI MB's.  

Long as linux plays well with your MB, USB drives work fine.  Check out Clonezilla, which is a nifty linux timesaver.

I like AM3 sockets with inexpensive, low power sempron or athlon chips.  2g of RAM is enough.  

Tricky part about mining with the 290x is choosing the right power supply.  For a 4 GPU rig you'll probably need 1200w to get enough overhead, and enough connectors.  The 290x is such a beast, there's an argument for running 3 to a motherboard.  Get by with a less expensive PSU.  Just depends on what your various components cost.  Gotta do your own math but the bottom line is, don't skimp on your power supply, or on your risers.  Get the best you can buy, don't burn the house down.

290x is a great card for XMR but to maximize profit, consider mining to an exchange, mine whatever is most profitable at the time (ZEC is looking good these days) and then flip to whatever you want to hold, whether it's XMR, BTC whatever.  

Xubuntu 14.04 (trust me; I suffered so you don't have to).  Set up fixed IP address and DNS server ( 8.8.8.8 ) make sure internet is working.  Turn off updates once you set location and language.  Radeon drivers are in the repository.  Install your graphics cards one at a time.  Compile your miner. Install SSH server, set up public/private key authentication, turn off password authentication.  Get into grub and set it up so you boot to the command line.  Shazam!  I love this ****

Linux isn't too too hard.  Once you start setting up your network and SSH you'll see the beauty in it.  Xubuntu 14.04 is a straight up vanilla, lean linux install that works like a charm right out of the box.  It practically installs itself.  Figure out how to use the Debian/Ubuntu repositories.  There are some excellent articles on setting up SSH over on the Ubuntu forums.  After that you'll find the best documentation over on the Arch website, believe it or not.

OpenCl goes in /usr/lib.  Just sayin.  Someday you'll thank me for that.  

GTX 1080 in my view

It depends alot on the memory manufacturer of your card. For example i had a few 7950's with Elpida memory and Hynix. The ones with Hynix have like an 80-120 h/s advantage over Elpida.

Same story with Rx 570's