I have a 64 bit AMD chip and I'm happy with it. I have a low range one, but it performs much better then my old 32bit AMD I had. The real difference you get tho is when using XP x64. It has a remarkable difference then on a 32bit OS. The only problem is the drivers tho. I have x64 running as a second OS and the only thing I haven't yet got to work was my HP printer and my webcam. All the other hardware works fine with the various drivers available.
The other problem is that lots of applications is still not 64bit ready. Go check here
Windows 64bit Compatibility Guide for software/hardware that is supported. I think this link was mentioned here before.
It's not really aswering your question, but hopefully it's usefull info.
Cheers,