Thanks! Is there a big difference between 4 gig cards and 8 gig cards? I can get an 8gig version of this card for about $60 more...
Also, if I wanted to do 2 GPUs would I be better off doing 2 of the RX580 cards, or keep the on 580 I have, and add a GTX 1070?
$60 more is way too much to pay for 8GB cards. I usually pay ~$25 more for 8GB cards of the same brand and model. 8GB GPU's usually will have faster memory than 4GBs, but for the most part, there is not much difference in hash rate between 4GB and 8GB GPU's of the same model. The benefit of 4GB cards is you can buy more cards for the same amount of money, so you will get a better hash/$ performance. The main benefit with 8GB is slightly better dual mining performance, more overhead for future Dag file growth on Ethash coins like ETH/ETC and better future resale value.