Thanks for the reply cmn.
Actually the MOBO that worked was a Abit AT8 CrossFire edition.
And seeing that 7 was not installing, I tried installing Vista x64, and it installed without any problem... So there must be something on the MOBO that hangs 7 installation. Not sure what yet.
EDIT (Update): just downloaded build 7068, seems to be working now (at least I am at the "Expanding files" phase). Maybe a BIOS / 7 build 7000 incompatibility that MS fixed in last build... Now if only I could upgrade bios to test this theory
Update : installation did work all right... But computer is freezing as soon as 7 build 7068 starts, at the exact same point (after first pixel drawing of the new "Windows starts"animation)...