I too have been comparing OCZ GameXStream 700W with Enermax Liberty 620W.
The OCZ 600W does not supply enough amperage to feed those 4 rails - the math says there are only 12 amps for each rail (although any 1 rail is rated @ 18A max.)
The OCZ 700W increases this to 16A x 4 (all 4, continuous).
I find the Enermax 620W attractive, with 2 very stout 12V rails and the modular cabling.
If I wanted 4 rails for dual-core/dual-vidcard, I would select the OCZ GameXStream 700W model.
I love the OCZ ModStream - it is fantastic, a work of art but its day has passed...
The Liberty only comes in 620W. I would go with the Enermax Liberty over the OCZ. Enermax makes more reliable and stable psu's. I would prefer the Enermax Noisetaker 2 (600W). The Liberty is amazing because you can hook up to 18 HDD's to it and it will run flawlessly. I've only hooked up 6 HDD's to it and I've never had any problem.