Experiences with 64 bits AMD processors

we are interested in buying some more computational resources. In our group we are interested in 64 bit AMD processors, but we do not know about their compatibility. They are supposed, as AMD says, to be32 bit compatible and therefore AMD 64 bit processor should be able to run any 32 bit application. Is that true? Any experience about this will help us a lot. By the way, we are running mainly gaussian jobs, and have some other 32 bit binaries like turbomole and jaguar. We have source code license for gaussian 03. Has anyone tried to compile Gaussian 03 for a AMD 64 bit machine? Do 32 bit pentium binaries run correctly on a 64 bit processor? which is the increase on the performance? Do Turbomole and Jaguar binaries run on 64 bit AMD processors? anyone tried?
 Any information will be helpful.
 Thanks a lot
 Marc Noguera Julian
 Thcnic Especialista de Suport a la Recerca
 Qummica Fisica, Universitat Autrnoma de Barcelona.
 Tlf: 00-34-935812173
 Fax: 00-34-935812920
 e-mail: marc_at_klingon.uab.es