I disagree - subsequently agreeing with Kolgar - as I believe those people that got a N64 and not a GameCube would have jumped on the PS2 wagon if Xbox never came abouts.
N64 was quite a different scenario. It launched later than the PSone and was an overal good competitor for a very long time. Games like GoldenEye and Perfect Dark consequently held the perception that Nintendo was in fact following Sony's maturity content - which of course in the long run never really happened - but at least belief was strong which is why, coupled with the fact that Nintendo was the marketleader, they managed to stay competitive for a very long time.
After the disappointing outcome of the N64 and those fans would have probably moved on to the next best thing available: the new marketleader Sony. As a subsequent, Nintendo's userbase would have decreased either way, regardless of MSs entry.
It's quite evident that both Sony and Microsoft are offering similar content and as a result are targeting similar gamers. Actually, there's still one thing MS hasn't: Family games for the younger audience. I think Sony is carving out more than one could think.