A while back I wrote an article for MSExchange.org on why 64-bit is good for Exchange 2007. 64-bit is good for more than just Exchange 2007 and SQL 2005, and this series will cover other scenarios that can benefit from 64-bit. Let's look at terminal services first.
Probably the biggest limiting factor to terminal servers today is the virtual memory limits of a 32bit OS. No matter how much physical ram is present, and how many CPU cycles are available, once you hit the 2GB virtual memory limit, performance takes a nose dive. By moving to an x64 platform we are freed from this limitation (up from 2GB to 8TB) and we get access to much larger amounts of Page Table Entries (PTE) and larger paged and non-paged pools. In all three cases the limit is raised to 128GB. Of course you also have access to all the memory in the server which in the case of Windows Server 2003 Standard is 32GB but in Windows Server 2003 Enterprise Edition, up to 1TB of physical memory is supported. With the memory limitations effectivly eliminated, disk I/O and CPU power becomes an issue. Support for 8-way systems will take care of part of this, but we have to wait for faster disk subsystems for the other half of the equation. So you are hyped and ready to buy an x64 terminal server now, you might wonder about all the users apps that are 32 bit. Not to worry the x64 platform and Windows x64 OS can run 32-bit apps thanks to the WOW64 transition layer. This is similar to how 16 bit apps run on current 32 bit Windows OSes.