i686 or AMD64 ??

Hi :)
Wondering of what is the best solution for my hardware setup, that is:
asus m2n-mx motherboard
amd64 3500+ CPU
1024 RAM (geil) dual 512
onboard vga (geforce 6150)

And then, what is the best to run with
i686 of AMD64 ??

Thnks vey much for contibutions
Thomas