Any of those options will work fine. (as long as the used Corsairs are in good working order).
The 550W recommended PSU was for an 'average' PSU. The Corsairs are much better than that.
I read it here http://www.tomshardware.com/reviews/geforce-radeon-powe... that my card need 25A 12v rail and my hd3870x2 can eat up to 200w when 3d . In this case Corsair 520HX will fail because have only 18A 12v rails and combined rating of 40A (what combined means? ) . So the best choise remain 550VX wich gives 40A 12v rails ?
The HX520 (also the HX620) is really a single-rail PSU. So don't have to worry about power distribution because you will have room enough for running your system with those 40A/480W on the +12V output.
The 550VX is better because is new (when a PSU is used heavily or over an extended period of time (more than one year) it will slowly lose some of its initial wattage capacity), performs very well and the price is not high.
thank you Henry Chinaski ! I didnt know that with time PSU slowly lose some of its initial wattage capacity . That 520HX was 2 years old . I just ordered brand new 550VX with 90 euro (bonus 4gb corsair stick ) .