I've got a P100 with 64MB ram running Debian sarge minimal install with Apache 1.3, PHP4, MySQL. Ther kernel isn't optimised in any way. I'm running mambo as an internal intranet CMS and a helpdesk/asset management system called IRM from stackworks (both PHP). How many users do you think it will take to bring this box to its knees? Should I be able to get 50 simultaneous web users getting pages from this box?

Anybody know where I can get guidelines / stats from or have similar experience running LAMP on low spec boxes?

Untill recently the Mambo server didn't use caching for their pages. The new version coming out now have a caching function built in which would speed up consecutive request of the same page.

You might try to load test it your self. There are special programs for that, but for a simple test you could use wget to download the entire site. This should give you a hint on how your server is performing, not in term of "users" but how it copes with heavy load. Monitor your server to see how it reacts. Check with top to see how your load average changes and how the separate httpd processes perform.

You could also test your response time during this load, with the ping command set to ping every second or so.

Also have a look at what the pros think of your site.
http://www.websiteoptimization.com/
They have a speed test service where you get information of how long time your page take to load and tips on how to improve that. If a web site is working is subjective, it depend on how long your users are willing to wait for a page. You will probably loose users long before your server gives up, especially poor dial-up users.