We suggest having enough RAM to run everything at once, otherwise you'll get swapping and that really slows things down. Depending on how many other services you run, 64 meg might get you by, but we'd prefer that you use 128 meg or more. There really is no set limit. Also, you will notice that a Pentium III 450 is faster than a Pentium 90, but SquirrelMail should run on nearly anything you have.
If your web server is choking, churning, and crunching the hard drive a lot, perhaps it is time to pop in more memory. Since SquirrelMail is a web-based IMAP client, all of these programs need to be loaded simultaneously.
- Web server (Apache, etc)
- IMAP (Courier, DBMail, UW, etc)
- MTA (sendmail, postfix, qmail, etc)
Depending on options that you have, it might also tie into a database, LDAP server, or other things. Each thing that is running simultaneously chews up more memory.
Another thing you can do is try to upgrade your hardware. Linux and FreeBSD will run on a 386, but will scream on newer computers.