Finally I chose a Windows XP Home 32-bit OEM and I don't regret it.
The games I tried so far were 'Tabula Rasa' and 'The Witcher'. Both of them run super-smoothly with all settings at HIGH, native resolution (1440X900) and AA turned off on my 2.4GHz MBP. I expected a decent gaming experience with settings at medium and the resolution turned down a bit and actually only went for everything on HIGH and native res just to make some screenshots... that the games would be running totally BUTTER was quite a surprise! The guys who write the drivers at Apple really know what they are doing, my respect for the coders.