Avalon ASIC PSU?

[BitAddict]

Hello, i bought one Avalon ASIC ~620W without PSU.

And I'm looking for buying one PSU. Any advice?
750W should be ok?
Silver, Gold, Platinum certificate? or just normal one?

For me its really important expending just the minimun electricity because it's expensive in Spain. But I don't now if its worth it.

Thanks!