Assuming you are mining ETH, 1000 watts will be enough to power 5 x 5700 XT plus MB, CPU, etc. but only if you undervolt the GPUs. If you wanted to add more GPUs in the future you can get a little device called add2psu to connect the 1000w and 850w PSUs together. One thing to keep in mind though is that If you are using risers you need to power the riser and GPU from the same PSU.