An 80 Plus Gold PSU is going to cost you about the same as a bronze or silver 1200 w PSU. You'll also be running the PSU at 80+% capacity 24/7 ... most PSU operate most efficiently at ~50 - ~60% load, not 80%... so your cost over time is going to be higher running a 1KW PSU as well.
While I don't believe any of my sub 1.2KW PSUs were 80plus GOLD rated, they were quality PSUs (Corsair, etc) and they had stability problems if I put in more than 2 5970's along with 5870's. Moving to a 1200w let(s) me put 3 5970's and 1 5870 on the same rig. Anything less and the rig becomes unstable. Swapping one of the 5970's with a 5870 on a sub 1200w PSU and things become stable again.
Like I said, maybe an 80 plus gold would be different, but for the cost difference, my money is on 1200w.
Interesting alternative view. It is amazing how important the PSU ends up being. Then again we are pushing these machines far beyond what they were ever intended to do. Maybe i just go lucky w/ good PSU. A lot depends on not just the wattage, but rail configuration, quality of the regulation etc.
One PSU might have nearly perfect regulation @ 60% load but goes to crap at 80% load.
Another might have more variability in voltage @ 60% load but isn't much worse at 80% load.
For the record my rigs all have 3x5970 running on either:
1000W model
http://www.newegg.com/Product/Product.aspx?Item=N82E168171710561200W model
http://www.newegg.com/Product/Product.aspx?Item=N82E16817171055My workstation (4x5970 water cooled) uses this beast:
1350W
http://www.newegg.com/Product/Product.aspx?Item=N82E16817194092A little too expensive for mining rigs but I use the workstation more for just mining. It is an awesome PSU the only con would be the price. Nice thick heavy cables, quality cable wraps, all the cables are modular (including ATX 24 pin) which is useful when you need to get into that case crammed w/ 4 dual GPU cards and water cooling gear.
The one thing Bitcoin has taught me is that PSU are the unsung heroes of the computing world. Quick question do you run your rigs @ 230V? Either at home or in your datacenter. I am thinking about maybe running a pair of 230V 30A lines in my garage to squeeze another 5% or so efficiency out of the rigs as well as move them all to dedicated circuits.