not correlated in this respect means you have full additivity?
I want to start with 3 GPUs. Would you recommend me to buy a 1500W PSU right away or buy a 1000W now and 500W later in case I want to upgrade?
If you have some basic soldering and electrical skills, check this out. High quality, very inexpensive 12v power supplies for GPUs. I got 1200 watts for about $60 and about an hour of time.
http://www.xodustech.com/projects/bitcoin-mining-farm