How to level two different floor heights to prepare for carpet?
I'd probably do 1/2" sleepers (could be ripped from plywood or OSB) directly over the joists, then 3/4" t&amp;g subflooring with construction adhesive between everything to prevent squeaks. Use screws or ring-shank nails long enough to penetrate the existing subfloor and joist at least 1-1/2". 

Weight shouldn't be an issue, but I know nothing about your home's construction.