[Ilugc] [slightly OT] 64 vs 32 bit processors

[km]

hi all,
we are planning to migrate to 64 bit architecture from 32 bit for scientific computing for our servers.
i need a suggestion or remarks abt AMD Opteron series  vs Intel Xeon processors.
As most of our applications are memory intensive and 32 bit processor based, and since AMD Opteron series talks abt simultaneous 32 and 64 bit functionality, i want to know in practice if any issues not to be overlooked while migrating to it.
In this connection, which GNU/Linux  flavour is best suited for 64 bit computing ? any recommendations ?

advice appreciated.
regards,
KM