When you have a 64 bits processor you can use a 64 bits version of linux for taking advantage of the full possibilities of your processor (all distributions have a 64 bits version).
However there are some differences with 32 bits distributions, most often due to packages that don't exist for 64 bits.
Unfortunately there is no 64 bits Adobe Flash plugin.
The first possibility was to install 32 bits binaries of firefox to use 32 bits plugin. The problem is that it is possible to launch simultaneously a 64 bits and 32 bits version of firefox, so that you had the choice between using always the 32 bits binaries (which is a little bit frustrating), or closing all your sessions of firefox when you want to use flash and launch the 32 bits version.
But now there is a solution :
nspluginwrapper which, as its name suggests it, wraps 32 bits plugins to launch them in your 64 bits browser.
Moreover, it is difficult to make simpler :
emerge nspluginwrapper nspluginwrapper -i /usr/lib32/nsbrowser/plugins/libflashplayer.so
I tested it with flash7 and flash9 and it works just perfect.
For a long time I experienced a sound problem with Amd64+Flash+ndiswrapper: flash was stealing the sound card and never releasing it, so that I had no sound in Flash if I was using sound in other software, and once I used sound in Flash I couldn't get it back in other software. The only solution was to restart alsasound service (
/etc/init.d/alsasound restart), that was killing Flash with all other software using sound, but if I wanted Flash back I had to restart firefox. Pretty annoying.
After a long search, I ended finding a really simple solution here: just rebuild