Minimum wattage for a build that is required for stability.

[Erich66]

Hi, my customer has a build consisting of:
CPU: Fx-9590
Gpu/s: R9 290
Motherboard: ASUS Sabertooth R2.0
Storage: 1x Seagate Barracuda 1TB, 1x Samsung SSD 840 evo ~250gb, 2x caviar GP 500gb (RAID 0)
PSU: Corsair CX750M
The customer wants a upgrade to the latest technology so he can get even more fps in games and I'm thinking of changing the PSU to a 900watt-1000watt one and putting in 2x GTX 980s by MSI. The budget is not a thing of interest for him so the real question is: will the system be stable at high loads?
P.S. To think about saving a bit of money, what would be the minimum wattage needed with a 150watt headroom?

 

[Agera One]

750W is enough for a system with GTX 980 SLI: http://www.realhardtechx.com/index_archivos/Page362.htm

With 150W head room, 900 W is the number you've to get. Make sure you're getting a very quality unit for the build. You can choose one from here: www.tomshardware.com/forum/id-1804779/power-supply-unit-tier-list.html