You're kind of limited by the fact that the PPC part of the Mac OS X ecosystem is pretty much a dried up withered husk at this point.
Honestly though, I would think that your standard router firewall would probably be enough for the task. A personal web server is likely going to be so low volume that it will never really attract the attention of any ne'er do wells. So you just need to be able to ward off automated scans, and a router firewall would do that just fine.
You also need to make sure that you keep your web server software up to date. This may mean that you have to set up something like Darwin Ports and compile your own custom Apache build. If you want people to be able to access your web server from the Internet, then that also means that automated scans can be run on it, and so if you're running a version with some known exploit, no firewall software in the world is going to protect you. If you're only going to make the web server accessible on the LAN, not the WAN, then you don't need to worry about that as much.
Basically, when you get down to it, about 90% of computer security comes down to the user not doing something stupid. It's not about buying some $2,000 IDS device or really expensive networking gear, it's about you the user/administrator only turning on and exposing those services which actually need to be on/exposed, making sure software is kept up to date, and actually fairly simple things of that nature. You will never stop an honest to god live person hacker if they want to get into your system, but you can make your systems look like more effort than it's worth to break into compared to the next guy.
I am going to use my old G5 as a web server. It is running 10.5. Attached to my WFI are two computers running Lion. I need a program to help protect the server and my tow computers from being hacked. Any suggestions?

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