gnu/linux on AMD: 64-bit or 32-bit?

[kirblam at comcast.net (John Kirby)]

Friend of mine just mailed me. He got a swank new machine and was curious 
about 64bit vs 32bit versions of distributions:

"My processor is an AMD Athlon 64. This means I could opt for a 64-bit version 
of Linux -- I have seen several. I'm fearful though that I may end up with 
something less tested and hence less stable than a plain old 32-bit version.

What do you think?"

I only have experience with RH and Suse 32-bit installs so any feedback I can 
pass on is appreciated. 

Thanks
K