Keep in mind the reason every AMD video card is sold out right now is because EVERYONE is building rigs.
The total hashrate in ETH equates out to appx. 1 million AMD GPUs - which isn't even in the BALLPARK of their total sales on the RX 470 and RX 480.
If you add in ALL of the hashrate on everything else, it MIGHT double that figure.
Then factor in a LOT of older AMD gpus are mining ZEC, some are still mining ETH, a few mining other stuff, then subtract out the NVidia brigades....
AMD cards are not in short supply due to mining (though it is a small factor, there was NO shortage on the RX 4xx series for many moons up to last month when they were discontinued), they are in short supply 'cause the RX5xx lines haven't gotten up to full speed yet *AND* AMD is selling MAINSTREAM cards that perform comparably to many of the high-end cards of the last generation for well under half the cost while eating half the power *AND* Nvidia has been very slow to move DOWN to the mainstream segment on their current generation cards.
I also suspect AMD has been a bit slow DELIBERATELY on getting volume production and shipping going on the RX 5xx series so the remaining RX 4xx cards could get cleared out, since they don't have a lot of competition YET in that segment.
I highly recommend AGAINST Poloniex - I had nothing but ISSUES far too much of the time I dealt with them over the last month and a half or so. They seem to be suffering some SERIOUS "can't handle their growth" pains.
I don't like how Bittrex has their website set up in some ways (their pop-up "order confirmed" RIGHT on top of the main menu is VERY irritating, as is their insistance on using Crapcha) but they do get the job done and don't lag to hell and gone for hours a day like Poloniex does most days - and they don't hang on to your withdrawals for DAYS at a time.
Never used Kraken, GDAX, or Bitfinex - but I am probably going to be checking one of more of those out in a month or so (I have a move tentatively set up next month into a MUCH better place).
For getting your fiat into / out of BTC/ETH/LTC I find Coinbase works reliably, though their ATH transfer times are a bit slow that seems to be true for everyone that uses ATH.
Poloniex and Bittrex have NO option to deal in fiat at all.
See my last post for *MY* estimate of when the current very nice profitability will come to a screeching halt.
IMO LINUX is the way to go for mining, though it would be nice if it had better tools like Afterburner for tweeking cards with.
Open air rigs are usually a hair noisier, but you don't need MASSIVE fan volume on them like a "in a box" rig does to stay cool so sometimes they're quieter - and they're almost ALWAYS cooler.
They are also a little less expensive (no case cost), and they use a little less power (no case fans needed, or you can get by with a MUCH LOWER POWER fan at most).
