64-Bit is the way to go...
Yes, it's time to move to 64-bit MOSS if you haven't already. Here are a few points that I have been sharing with others.
1. The next version of SharePoint will be 64-bit, no 32-bit options, so plan for your hardware refresh now if you haven't already.
2. Taken from the Best Practices for Operational Excellence white paper:
Use 64-bit hardware, plenty of memory, and fast network cards
64-bit hardware (for all server roles) offers the best performance for Office SharePoint Server 2007. In addition, make sure that you allocate appropriate memory for each server role:
Allocate at least 2 gigabytes (GB) of RAM per processor for front-end Web servers and application servers.
Allocate at least 4 GB of RAM per processor for database servers.
Use gigabit network cards for all server roles.
For front-end Web servers and application servers, use dual NICs in production environments: one for users, one for SQL Server communication.
Under heavy load, consider using virtual local area networks (vLANs) to reduce network traffic.
3. A MOSS mixed architecture is ok. For example your SQL database and Index server could be x64 while your WFEs are x86. Improved Indexer performance is always a plus, and leads to observable performance improvements as long as the network and storage systems aren't the bottleneck, at least based on what I've seen in real customer environments.
4. Several benefits with with the x64 OS vs x86, see "Benefits of Windows x64 editions."
5. This article does a real good job on "When and Why should I move to 64-bit computing with SQL 2005?", and the advantages of a SQL 64-bit environment.