During peak loads, Apache will spawn more processes and use RAM that the database server might have been using, or that the kernel was using on its behalf in the form of a cache. You will starve your database of resources at the time when it needs those resources the most.
Disk I/O contention produces the biggest time issue. Adding another disk won't cut I/O times, because the database is the only thing that does I/O—mod_perl processes have all their code loaded in memory (we are talking about code that does pure Perl and SQL processing). Thus, it's clear that the database is I/O- and CPU-bound (it's RAM-bound only if there are big joins to make), while mod_perl is mostly CPU- and memory-bound.
There is a problem, but it doesn't mean that you cannot run the application and the web servers on the same machine. There is a very high degree of parallelism in modern PC architecture. The I/O hardware is helpful here. The machine can do many things while a SCSI subsystem is processing a command or the network hardware is writing a buffer over the wire.
If a process is not runnable (that is, it is blocked waiting for I/O or something else), it is not using significant CPU time. The only CPU time that will be required to maintain a blocked process is the time it takes for the operating system's scheduler to look at the process, decide that it is still not runnable, and move on to the next process in the list. This is hardly any time at all. If there are two processes, one of which is blocked on I/O and the other of which is CPU-bound, the blocked process is getting 0% CPU time, the runnable process is getting 99.9% CPU time, and the kernel scheduler is using the rest.