While load testing MOSS at the Microsoft Technical Center (MTC Lab) in Waltham, MA we confirmed that mixing 32 bit and 64 bit web front end servers will not work. When we created web applications on the mixed MOSS farm we found the timer job for provisioning the sites to the other web front ends would fail. We noted the "stuck" provisioning web application job under the operations tab in central administration. In addition when we deleted a web application the unprovisioning job never executed.
Since Microsoft will not be releasing the next version of MOSS for the 32 bit platform I advise using a 64 bit architecture if you are implementing a new farm and have this luxury. In an existing farm use caution when mixing your environment. Note that a 64 Bit database server, 64 bit application server and 32 bit web front end servers (all web front ends must be the same) is supported.
Another consideration when moving your index server to 64 bit architecture will be the ifilters. You will need to purchase third party ifilters for PDF, etc. The current ifilters for Adobe are developed for the 32 bit code base.
