I came across a post on the lazy admin today about the benefits of x64 architectures for Active Directory. Although the arguments for AD are similar to those for Exchange (since they both run the same database underneath), the advantages for AD are not on par with those of Exchange. I agree that in large AD environments the ability to scale the memory to 32GB+ is great and will improve local performance of the single server in question. The piece that this should be weighed against is the cost of an additional server to process those additional requests (and act as another redundant node). Are there any price comparisons available for this that really show the $3,000 for 16GB of memory will provide that much of a benefit?
I know that Ed Brill and Paul Robichaux are talking about this same question for Exchange. I have priced these options myself for large Exchange environments as well and there are some real break even points and advantages on both sides.
For a small environment, it looks like the benefit would be greatly reduced from all sides. The cost to purchase an additional server has more advantages then the same cost to purchase more memory will provide (redundancy, etc).
SMB Customers should be cautious in this space where larger customers who are already running into authentication or paging issues should investigate all of the options.