How much can a G5 Xserve handle?

Is there any way to figure out how much a server can handle? I have a 2GHz G5 Xserve with 2GB RAM running 10.4.11 Server at one school... We just had to downgrade from 10.5 server because of all of the AD/OD problems. Over the summer, we swapped the PC lab in the building for an Intel iMac lab and added another cart of 15 macbooks. That gives us two labs of about 30 Intel iMacs each, 45 macbooks floating around, and two iMacs per classroom (30 classrooms). These are all running network home folders with the ~/Library/Caches folder redirected to /tmp on the local hard drive. The server has been crashing a lot lately towards the end of the school day and they're saying it's when the carts are out and both labs are being used. When it happens, it takes users several minutes to logon and those who are logged on get the spinning beach ball and can't do a whole lot. AppleFileServer is using a good percentage of the CPU, but it's not totally maxed out the whole time. This is pretty much the exact same setup we have for 12 schools and it's just this one school that is having problems like this. I'm just trying to figure out if this server is getting overloaded, and if so, is there anything I can do about it to try and lessen the load (before buying another server)? thanks, JB