I have the 750W mounted upside down in my cosmos S running raid etc etc, hasn't dropped a beat. The 1000w unit is big and grunty, have you worked out how much power your going to need? Most dual SLI setups still only use 750W.
i'm not sure what to go for now. articles say 1000W isn't enough for gtx sli (which i had hoped to have in the future at some point, hence the over kill psu). now i'm thinking of going smaller to fit today's requirements since i know i'll need 1200W to run gtx 280 sli.