Scalability Experts have recently published a great white paper (PDF) that compares SQL Server 2000 running on 32-bit and 64-bit architectures and benchmarks both versions against known loads. The whitepaper uses a Unisys ES7000 mainframe-class server using Itanium 2 processors. The most interesting results were using the 64-bit edition of Analysis Services, where they demonstrated far higher headroom in terms of number of members and partition processing. Some of the most severe limitations of the 32-bit edition will be eradicated when SQL Server 2005 arrives, due to the rearchitecture of much of the OLAP engine, but the enhanced throughput of a 64-bit environment will still have a major effect.
The paper doesn't go into deep technical detail on how the benchmarks were conducted, which is a pity, but if you're looking for a broad technical overview of how the introduction of 64-bit will improve SQL Server scalability, it's not a bad read at all.