Probably XP :L
Did some testing with ATI presets with SS08 at highest possible including 20KM view distance:
Test Ship = P&O Ferry
OceanOptimal Quality - 30FPS
Balanced - 60FPS - Keeps at 60
Optimal Performance - 60FPS - Keeps at 60
Phi Phi IslandsOptimal Quality - 27FPS
Balanced - 47FPS
Optimal Performance - 46FPS
HamburgOptimal Quality - 20FPS
Optimal Quality - 26FPS - Without Max (Set to high)
Balanced - 23FPS
Optimal Performance - 25FPS
Note I'm in a dence part of hamburg with loads of containers