Microsoft has for some time given as a general rule of thumb the guideline that you should have 1 Global Catalog processor for every 4 Exchange server processors (1:4).  But that was before we had a 64bit option to run GCs...

We are seeing internally that we can run closer to a ratio of 1 64bit GC processor for every 6 or 7 32bit Exchange 2003 processors when we run our GCs with a 64bit OS instead (1:6/7).  Why?  Since we have more memory that is usable we can load the entire AD in cache if needed.  With a 32bit OS we could only cache at the most 2.6GB of the AD, but with the 64bit OS we can go up to many terabytes if needed.  Our AD is larger than 9 GB so you can see why we would go 64 bit instead.  If your AD is large then you may want to consider the usefulness of upgrading to 64bit as well.

Take a look at the following post for more information:

http://blogs.technet.com/windowsserver/archive/2005/03/10/392935.aspx