CNet News had the opportunity to talk with AMD's chief technology officer, Fred Weber, who has been responsible of the big success the Athlon XP is/was and most recently managing the strategy under which AMD has commercialized all hybrid 32/64-bit CPUs, here's a nice quote from the interview:

It took quite a while, though, to get it out into the market.
Well, you know, it is much like any of these developments. You are juggling a lot of balls all at once. We had a number of software efforts that we had to get done, and all of this was happening in parallel. It wasn't just inventing the new instruction set architecture, AMD64, but we also had to invent HyperTransport and develop onboard memory controllers.
By the time you get out a decade or so, certainly, 64 bits will be appropriate, even in what we think of as the smallest devices today.

At the same time, we were still fighting through the DDR versus Rambus nightmare. That was yet another fight in which we had to take a different path from Intel and fight uphill for what turned out to be the right final direction.

All of those balls were in the air simultaneously. We had to create simulators to allow Microsoft and the Linux community to port their operating systems before we had hardware. Somewhere in late 1999, I pulled together a team of about 20 key people and took them all out to dinner, and we declared that we would work together to make this thing happen.