There're a couple of solutions I can think of.
Depends on what you want to use it for.
You can either setup linux on another PC and configure it to output to serial and then move the drive to the PDS, you can add VGA to your PDS, or you can install m0n0wall on it.
The easiest...
Is to setup linux on another computer and configure it to output to serial.
This may vary from distro to distro.
I personally used slackware.
Here's a site explaining changing the output to serial.
http://www.techfinesse.com/wdupre1/net4801/SoekrisSlack.php
Should be able to follow step 5.
I tried this at first, but didn't stick with it for long.
I chose to put VGA on my PDS. (I like seeing things)
Here's pictures of someone else's PDS.
http://www.nullsense.net/Gallery/index.php?dir=/Projects/PDS2315
I actually de-soldered a vga connector from an old video card, soldered it on, cut a hole in the back panel and made it look like it's had VGA from the factory.
It's rather simple if you know how to solder and it works great!
But be warned, you can screw it up!
One more solution..
If you're planning on keeping this PDS to use as a firewall, you can load up m0n0wall (
http://m0n0.ch/wall) on a CF card (or hard drive).
From what I've seen, all the PDSs have built in CF sockets (FYI, Secondary slave).
It fits on an 8mb CF card.
This is what I'm currently running on my PDS. (Generic PC image)
I'm not sure if VGA is required to install this as I had already soldered on the port by the time I found this software and don't remember if I used the VGA port or not.
I've been running this for a few months now.
This is the best solution I've found so far.
It does damn near everything I want and it's easy to setup.
Hope this helps, if this seems like too much work or you just want to get rid your PDS, let me know.
I'm interested in getting another PDS or two for other projects.