The solution is to add another machine, which allows a setup where both the database and the web server run on their own dedicated machines.
This solution has the following advantages:
If your httpd processes are heavily weighted with respect to RAM consumption, you can easily add another machine to accommodate more httpd processes, without changing your database machine.
If your database is CPU-intensive but your httpd doesn't need much CPU time, you can get a low-end machine for the httpd and a high-end machine with a very fast CPU for the database server.
It also has the following disadvantages:
Basically, you can have almost the same client-server speed if you install a very fast and dedicated network between the two machines. It might impose a cost of additional NICs, but that cost is probably insignificant compared to the speed improvement you gain.
Even the normal network that you have would probably fit as well, because the network delays are probably much smaller than the time it takes to execute the query. In contrast to the previous paragraph, you really want to test the added overhead here, since the network can be quite slow, especially at peak hours.
How do you know what overhead is a significant one? All you have to measure is the average time spent in the web server and the database server. If either of the two numbers is at least 20 times bigger than the added overhead of the network, you are all set.
To give you some numbers, if your query takes about 20 milliseconds to process and only 1 millisecond to deliver the results, it's good. If the delivery takes about half of the time the processing takes, you should start considering switching to a faster and/or dedicated network.
The consequences of a slow network can be quite bad. If the network is slow, mod_perl processes remain open, waiting for data from the database server, and eat even more RAM as new child processes pop up to handle new requests. So the overall machine performance can be worse than it was originally, when you had just a single machine for both servers.