Check out this, it should answer most of your questions...
http://www.gentoo.org/doc/en/gentoo-amd64-faq.xml#perfup
> Hi,
>
> My box is a working and fully configured Gentoo system, which is
> uptodate.
>
> For the sake of being able to address more RAM and for more
> calculation power (mainly for rendering purposes) I want to
> migrate to 64bit.
>
> I googled for some tutorial but found nothing appropiate (one post
> asked for the downtime to be expected while migrating a server --
> something which not applies to me...).
>
> My questions are:
> 1) Is there a performance gain, when migrating to 64bit if the
> target applications supports 64bit?
> 2) Is it possible - if( true ){ how(); } - to """simply"""
> "convert" a 32bit system to 64 bit.
> "Simply" in my case means: Simpler ways than starting right
> from the bare metal of a virgin harddisk and doing the same
> stuff I did for the current system again... ;)
> 3) Is there some tutorial, which show me the path to go?
>
> Thank you very much in advance for any help!
>
> Best regards,
> mcc
>
>
>
>
>
>
