Upgrading to an RX 580

I've been thinking of upgrading my now-old GTX 750ti, while on a budget, to an RX 580 8gb

I chose that card because of it's price and performance, seems cheap for what it is, and my wallet has seen better days

My question is, which one model should I get? Sapphire? Whats the difference between Pulse and Nitro? Pulse seems cheaper, but at what cost?

How different is the 580 from the 570 and 590?

I never really cared for high resolutions, I've been playing on 768p my whole life now, I'm guessing I should be able to run almost all new games on that?

This is my first time purchasing anything AMD, is there anything I need to know beforehand?

Thanks,