I believe that the 850w has a ripple problem of somesort.
However the 650w and 750w are the exact same thing as the Corsair 650tx and 750tx except they are modular and don't get Corsair's warranty. If you look they are the EXACT same price (with the same MIR of $10 too hrrrrmm

)
Still my friend wants modular and thats a good choice for him.
There are 1000 and 1200w models and they are probably decent but i know that the 650 and 750w are great choices.