ok i'm running FC2 x86_64 on an Athlon64. (ok so far)

question: if i were to install a software, and the only options available were i386,i586,and i686 (no x86_64 and in rpm format) which would be best?
and if there were an option for a source binary instead, would i be better off using that instead? coz i heard that i could get a source binary, then recompile it for x86_64 or something...
some help plz, guys

plz b patient, i'm new to this 64bit linux thing... :o