Check the games you are gonna be using (if its mainly flight sims) actually support SLI/multi GPU - for a long time FSX did not work with multi-GPU and not even sure it scales that well now due to the CPU useage - and its not uncommon with sim games not to work well with multi GPU. So the 590 or 400/500 series SLI may potentially be a waste of money.
Also personally I don't have a high regard for Asus motherboards (tho I have no problem with Asus as a company) but their motherboards have never inspired me with confidence, my money would probably be on gigabyte or evga.
Also personally I don't have a high regard for Asus motherboards (tho I have no problem with Asus as a company) but their motherboards have never inspired me with confidence, my money would probably be on gigabyte or evga.