In a site that has multiple servers (some top-of-the-line, others border-line) is it better to use PSTs (personal file folders)? In other words, does it enable Exchange to perform better if little-to-no mail is stored on the Exchange servers?
Repeat after me: "In an Exchange environment, PSTs are bad."
Say it one more time for good measure!
The primary purpose of an Exchange server is to store and process messages. There's no point in having it if you're not going to use it. PSTs have many disadvantages over the Exchange mailboxes, not the least of which are that they're harder to back up; they can be somewhat less reliable; they are not accessible from multiple machines (under most conditions); they take much, much more disk space to store the same amount of messages; and they are much harder for you to administer.
So, in a nutshell: no. The only appropriate use of PST files in an Exchange environment is for archive files. Let your users archive aggressively, give your Exchange servers as much RAM as possible, and try to locate Exchange on its own server whenever possible.
You might also do a careful rethinking of your site strategy and evaluate if you really need multiple servers or if you might be better off consolidating some of the users/mailboxes onto the more powerful machines.