I know a person was working on the benchmark for large system in a big computer company. Probably 10 years back some server just started using 64 bits CPU. He was doing the benchmark comparsion between 32 and 64 bits apps. The result is the most of 32 bits apps are faster unless apps does some special 64 bits optimization.Only those really need 64 bits apps, such as graphcs intensive, or large database.
Things might be changed because 64 bits compliation may already built-in 64 bits optimization, just recompliing source code with 64 bits complier may improve the performance(but if this is the case, it also will use more memory).