SLI / Crossfire is excellent if running HUGE resolutions or multiple LARGE format LCD / TV's
Resolutions like 6000x5000 or some junk
If you are running under the resolution of a single cards capacity SLI / Crossfire really does not...
Then you need to install an OS and it will boot
Your BIOS is LOOKING for a BOOT DEVICE!
It cannot find one because you have not installed an OS
Once you install an OS it will then not display that error unless the OS is not properly...
Should be fine.
6970 and 50 use right at 135 - 141 at 100% load
So say 300w to be safe
Then you HDD's, RAM, Optical, Fans. Lights, and other - may draw another 300w
Your PSU should be just fine
As others have stated your PSU is not...