It's now working !
A friend of mine (well, this one is a reaaaaal good one!) lent me his Intel845PE based mother board and everything works just fine.
It seems hard to believe but Geforce4+2.4.20+Sis645 is a non functional combination.
By the way, I'm quite deceived by this forum. I just have to look at the number of downloads of the files I've posted to see that my problem didn't interested many of you... I'll look twice before I buy an other nvidia product as this lack of real support has really been painful. Would you buy a product you know you'll have to wait 2 weeks before learning that you can spend 140 euros more to have it working ? never again.