PSU.Suitable output

You only need 650w, 750w to be on the safe side. If you want the very best in PSU quality consider the Enermax Modu700+ Gold certified, or Seasonic's M12D 750w.

I'm not familier with either of those brands. Not seen them for sale over here.
The Corsair PSU is 80plus Silver rated and with a 7 yr warranty.
My existing PSU is a Thermaltake 560w and has been very good but it's getting near 5yrs old now and is not a modular unit.

Agreed, but Corsair is not the only brand. Try and look for those. They are at the top of the PSU pile so to speak. Seasonic also makes the PSU itself, the Corsair is actually made by Seasonic.

Found the seassonic brand. That model is (from my usual supplier) $10 cheaper then the comparable Corsair unit but with only 5yr warranty as opposed to the Corsairs 7.
Specs otherwise seem to be comparable.

Seasonic would have better voltage/ripple measurements. Is it the Seasonic M12D-II 750w? For a PSU, you need to find one that runs at 50% or so for peak efficiency (Silver in the case of the Seasonic) and one that has ripple measurements under 120mv, which the Seasonic (and Corsair) both achieve. In terms of quality, I'd go for Seasonic.

It's just a straight M12D-750w

This question is, can running an overpowered PSU hurt anything? The answer is, no - except your budget.

It is important to note that a PSU will deliver only what is asked of it. So if a computer needs 400 watts, that is what the PSU will deliver, regardless if the PSU is a 500 watt PSU or a 1000 watt PSU. And the PSU will draw from the wall, only what is demanded of it, plus over head due to inefficiencies. So in this case, the PSU will draw from the wall 400 watts, plus about 20% or 80w for about 480 watts total.

Now no PSU is totally linear. That is, no 80% efficienct PSU is exactly 80% efficient across it's entire range. And usually a PSU is less efficient at the extremes - that is, when barely working, and when stressed near capacity. And I agree that 60 - 70% is typically that ideal range (but that is not set in stone). You do want some wiggle room, not only for future expansion and aging, but also so the cooling fan is not running full speed, maximum decibels, full time.

So IMO, if the 1000W PSU is way over powered, it will not be running at peak efficiency most of the time. This means, it will cost you more to buy, and cost you more in energy costs every day thereafter. I have a Corsair 750 and am real happy with it. I think you would be too.

Both the 750 and the 850 are 80plus Silver rated with 7 yr warranty. I'd be happy with either.

I doubt anyone keeps a PSU for 7 yrs, but the choice is yours, and both are built by Seasonic anyway. In prefer to buy direct from the manufacturer, so I would choose the M12D.

I kept a PSU for over 8 years! It was an Antec and built very well. Still running today in a friend's second computer.

Seasonic, PC Power & Cooling, Corsair, CooolerMaster, Mushkin, Antec, OCZ, ThermalTake, FSP Group, Zalman and Xigmatech are all name brand top quality PSU units.

Be sure it's modular cabling which will help in routing only the cables needed and avoid clutter too.

In days gone by, the quality of the power supply was judged by its weight too, signifying quality transformer, heat sinks, filtering capacitors, etc., and it still holds true today.