I had the 64 bit install for a short period not long ago. From lxf68 dvd. Gentoo 2005.0.
Not long enough to really give a full account of but, the install went very well. It hardly took any time at all. The second stage install was just as good.
I've been meaning to put it back on and will once i get on top of a few niggling things that have been taking up far to much time of late.
The drift i get though is that in day to day usage a person probably wouldn't notice much of a difference. Perhaps a form of placebo effect may occure though
But with apps that do a lot of intensive number crunching, such as in image conversion/creation tools and the like, there should be a noticable improvement.
The 64 bit Gentoo iso (dvd) had 32 bit compatibility libs too -=- so -> go for it
I'm a big fan of of amd 64 bit cpus too btw. It's the best move iv'e made for years. Even to day, iv'e been compiling all day and it's damm hot where i am, and humid -=- 4:30 PM dst, the sensors program says 35 degrees centergrade. Room temp is about 30.