I have used some miners to mine different coins.
And I found that only profitable option for me is to mine monero with Graphics card.
I've been mining monero from two of my regular use desktop and now I want some dedicated hardwares.
I've decided to use cheap/powerful msi motherboards without any casings. (please comment if I'm doing it wrong)
I'll be running Linux from USB Flash drive (or is it a bad idea?)
And I've heard that AMD are better. (Let me know if I'm wrong, I have a wider option for NVIDIA)
But I've smaller AMD list to choose from.
EAH6870_DC2DI2S1GD5
EAH6770_DC2DI1GD5
HD77501GD5
R9290X4GD5
I know the latter ones are most powerful, are they as powerful for monero mining?
The 6870 is probably going to be more powerful than the 7750, although it will probably be less efficient.
https://www.google.com/search?q=monero+mining+hardware+comparisonI would recommend looking for recent results (after developers have had time to optimize their mining software), and being skeptical about obvious outliers (knowing that, as always, YMMV). Most cryptocurrencies have people working on mining hardware comparison lists.
Specifically what motherboard are you planning on purchasing? In general, any with several PCIe slots will do, but some motherboards can make it a real pain to get several cards working. When I first got into mining, I used an old ASROCK n68s-UCC board with 1 x16 slot and 1 x1 slot. The x1 slot would only work with my GPU (through a riser) if I used a certain pin shorting hack. I haven't heard of any recent motherboards that require this, however. My first real dedicated mining rig used an MSI 970A-G43, which worked without any such modifications. I eventually retired it after ASROCK released their first H81 Pro BTC mining motherboards.
Running Linux from a USB flash drive may or may not work out. Back when I was mining LTC with my GPUs, I had Ubuntu 12.04 installed on a cheap 8GB USB 2.0 stick and it worked fine. I disabled swap on it to prevent unnecessary writes (cheap USB drives don't last super long). However, now that I'm mining Zcash, I use an old 80GB hard drive with Windows 7 because I need 16GB of virtual memory for Claymore's ZEC miner.
I don't know much about Monero or what you're using for mining software, so you'll have to do research to see if you need a large amount of virtual memory for your purposes. If you do, I would steer away from flash drives, or if you do use them, I'd recommend making a full backup image after you get it running (good idea to have backups, regardless).