Im using Win7 x64 ultimate and SHIII 1.4 +GWX 3.0
Played around the clock this game for 2 months and i didnt had 1 single CTD or any other glitch related to OS,( odities like ships going aground etc are our old friends..
), so YES it works flawless even on x64 version of WIn7.
I have to say here just 1 thing,it may be big thing, and that is
RIVERS!
it can realy ruin your day if u have bad drivers.