Nah two of these:
There is a nice article MikeC linked to showing memory really isn't a limiting factor for Sandy Bridge. The difference between 1333mhz and 2000mhz ram might be 1 or 2% in real world tests. http://www.bit-tech.net/hardware/mem...sandy-bridge/1
Then you should have plenty of ram for your needs.
That's a really nice setup once you get the ram issue figured out. I don't forsee and issues with Only
and 850W PSU. RageJG is running GTX 570 SLI on a 650W PSU. I've run 8800GTX SLI on a 750W before. Guru3d shows about 450W pull from the wall for 1 GTX 580 under load. http://www.guru3d.com/article/geforce-gtx-580-review/7
Your setup probably won't pull as much their their system does either.