Estimating Number of Users per Xserve

After spending some time researching OS X Server and Samba, I haven’t located anything that can help us scale our consolidation solution. I am looking for either a rool of thumb or estimates based on your experience about estimating the number of users an Xserve functioning as a file server can support.

We are thinking of using Xserves (Dual G5 2.3/2-4Gb) to consolidate a number of older Wintel fileservers. We will be using only SMB shares as the Xserves will be installed as Domain Members into a child domain in an AD environment with 95% of the client workstations being XP boxes. In the current environment, the load on the servers being replaced happens to be pretty low.

Now for some background on the infrastructure:
The network has a Gigabit backbone with 1 Gig to the servers and 100Mb to the desktops.

The Xserves will be connected to a SAN. There are several variables with the SAN. The SAN may consist of several Xserve RAID boxes or several DotHill SANnet II SATA boxes. We are also considering deploying XSan – only if it will assist us on load distribution by sharing a single volume between one or more servers.