Out of the box, the Opteron is a faster 32 bit processor. This means that ALL you existing software will work better on an Opteron. To take advantage of the full 64 bit power, however, you will need a 64 bit O/S and the app will need to be recompiled to support the 64 bit instruction set. 64 bit O/S's will be able to run all your 32 bit apps without change, but you won't get any performance increase over using a 32 bit O/S on your Opteron.

Almost any app will be able to gain performance by being recompiled for 64 bit mode, simply because of the increase in the number of physical integer and floating point registers, and the use of SSE/SSE2 for intrinsic floating point rather than the older X87, that the 64 bit compilers are using.