Having an i5 or faster and 8GB+ of RAM also seems to help.

I can get over 40sol/s on my RX480 in my i7 with 16gb machine. But in my dual core Pentium G3220 with 4GB RAM I only get about 30sol/s per card.

I've also noticed a performance boost by having the card in a 16x PCIe slot instead of 1x PCIe.  I would imagine PCIe generation differences also making a difference.