athon64 and porting problem

[lopio]

hi,
I'll like buying a new 64 processor bit and running gentoo.
I think about ahtlon64 fx ,what about motherboard and socket problem?
It necessary starting from amd64 iso,is it?
But how can I know which application in gentoo portage can be downloaded and compiled for this 64 bit system? It is not true that compiling for 64bit (using host flag) assicure an application works fine because it would suffer of porting problem (32 to 64 bit).
Can you explain it?
Thanks