does the GTS250 1GB green edition require a different PSU or not?
Whereas a normal GTS250 might well require a new (bigger) PSU, the Green Edition is designed to consume less power.
It will therefore be popular in small-PSU builds.
If you have a good PSU and are interested in performance, avoid the Green Edition which performs poorly (because of its below-spec clocking).
I am surprised this has not been mentioned to you,