I've been thinking of upgrading to php5 for the last few weeks now. I was reading that cpanel servers have to run mysql 5.0 along with php5 in order to work right. Are there really any advantages of php5 and mysql 5.0? I run a nice size forum that's growing at a good rate. By upgrading will I run into any performance issues at all? Any suggestions that might help?
You can only encounter difficulties, php5 might cause them. I suggest that you run a small home server and test if your forum will work on that offline server with the setup you mentioned (php5, mysql 5).
Another note - bottleneck of every website is database system. ASP.Net / Ruby / PHP are not the key point in the game of performance, it's the database (it's usually database, people who have no clue what they're doing can screw up and load huge chunks of data to their scripts, making them slow and blaming the language).
MySQL 5.x offers a new set of features such as triggers, views, stored procedures and they do improve performance (if you know what you're doing).
With those features, you can practically take off the load of your script and let database practically maintain itself, thus gaining performance. Also, stored procedures are faster than your "normal" queries.
So you *might* expect a bit of a performance gain, but it's not something that will suddenly turn to a jet-speed.
MySQL 5 shouldn't give you any headache when you upgrade, but of course - do your backup first. I had problems with cpanel before and it's upgrade system..
And of course, MySQL 5 is the most stable MySQL so far.