help me make the decision

i have a system that i use for mining when idle. currently running nicehash at 8-9Mhz using rx 570 4gb on windows. I'm planning to stop using nicehash once it gets to minimum withdraw threshold, and continue using other miner that mine ravencoin directly.

I'm using seasonic s12ii 520w 80+ bronze psu and planning to add one more gpu to it.

• can you guys suggest me what 4gb card should i get? since 8gb is almost impossible for me to buy.
• can my psu handle another card? I'm using i5 4590 cpu.
• according to nicehash calculator rx 470 4gb is more profitable than rx 570 4gb, is that correct?

I'm new to this stuff, and my english is not good. so please bear with me. thank you.