to be honest i prefer the old 6 rig setup with asrock h81 pro btc
with $500 you have a rig of six gpu
with the next gen gpu that consume less the perfect setup would be this for six gpu
asrock h81 pro btc
g.skill 8GB
G1840(you may buy a pentium here for only $10 more, it may be worth it for the extra cache ram)
ssd 64GB(120GB i've found one sandisk for only 40 euro on amazon)
evga 1k g2
total is 450 euro + 50 for riser you have your rig for only 500, then you just add your gpu
1kw is enough for future gpu because polaris and pascal will consume something like 120w per card, for gpu like 490 or 1070
Personally $500 is too much to build a basic rig. I like using older LGA 775 hardware and I can build a PVC rig for about $100 (minus PSU) so that roi can be reached much faster. It can be hard to find the proper mobo but when you do buy few of them. My logic is mining is a short term operation and roi needs to be reached asap. Its stressful watching difficulty rise and profit reduce when you have hardware on credit. After you have a steady income stream then it become easier to justify newer equipment
it's the psu that cost a lot, that build that i posted cost only 250 then you add 200 for the psu and 50 for riser, which you are obligated to buy, so it's very cheap in the end
buying parts that are too old can lead to compatibility issue, also
no motherboard is as good as the h81 pro btc for mining with six gpu