I'd vote for running a Quake 1/2 and or multiple game servers. Definitely would be period accurate and appropriate. To make things more interesting, maybe try NT 4 as the OS; that was an excellent file server for small and medium sized offices on fairly modest dual cpu systems back in its time. Windows 7 and higher don't have many issues with the older CIFS protocol in NT either. In contrast, you'll have one hell of a time getting a minimal Linux distro to run well.

I have 98se + XP3 on my single P-PRO @233 128mb of ram + 2 1TB hard-drives for storage.
Sil3512\3112\3114 Sata I controllers will let you use Modern Sata III drives with no issues under 9x.
You can use just about any PCI Sata II controller tho if you want to run 2k\XP\Linux.

if you want a REAL use for this system, and don't mind the power consumption, I would turn it into a router. In fact, when I first got broadband, I didn't want to buy a router, so I literally ran one on an old pentium. That grew to a pentium pro later. Keep in mind you need to find a SMP version of whatever router software you use, this is NOT hard. I used mine until I got 100mbit internet. It was replaced only because of bit-torrent, and just couldn't keep up with 100+ simultaneous connections. I use a dual p3 xeon now, and even it will be replaced when/if I ever get gigabit internet in my area.

along those lines, you can make it a secondary router/server. you can use it to isolate your pre-xp machines, from the internet and your new machines from the old. I have one of these at home, and I have an old laser printer connected and shared to it. I can print from any computer, even my old ones! I also have a 500gb hard drive in it that is used to store a lot of old software.

That said, my dual pentium pro sits pretty on the shelf at the moment. still works perfect.

Maybe W2k - much easier to get USB working that NT4, easier to get large disks working (that is if you want to put a bigger disk in that box.), also Direct3D. (In NT4 you should still get OpenGL and Glide.)
For gaming the Dual CPUs are probably rather useless - don't work in Win9x, and games that profit from multiple CPUs want much faster CPUs anyway. Nice idea about the game server, host a game and participate without too much of a performance impact.

If you want to experiment you could try to get OS/2 going, there should be an SMP capable version, and it's better for DOS games then NT/W2k.

At one point I had a Pentium Pro running debian acting as a router/firewall/dns and irc server. This was in the days of standalone DSL/Cable modems being commonplace. I had a lot of fun setting up all the services and configuring the machine.

Nice ideas rolling around in here so far.
Still not fully sure as to what i want to use it for but I know i will at some point just because its there to use. I have a CF adapter that I can throw in it as a slave and i will most likely up the main hard drive to something a little more modern but not sure if i need 200gb+ drives. If i go the storage route i will definitely look into the larger drives. If I can maybe tweak Ubuntu or go 2kPro, get the PCI USB to function I can then maybe get my 2 500GB externals to act as backup drives for the time being. Lots of options and regretfully i am not the smartest software user on the planet, I prefer hardware. Stick it together and make it come alive, the rest is just lets see 🤣.
Router does sound like a good use,along with the storage option. Not sure if it can handle the load of doing both. May need to get some pointers on the router setup option. Is it basically ICS or is it something else entirely? As i said, pointers for the not so software savvy =)
Thanks for all the post so far though, any more ideas are really welcome in the still broad spectrum of possibilities.

For a router setup, you'll want 2 network cards. One for your WAN (modem), the other going to a switch (LAN). The software will help you set that up. A distro like Monowall or PFsense is just a software version of a consumer router (gateway, routing, NAT, firewall). It can be higher performance than a cheap router. Not usually AC or prosumer stuff, but great for something made from old parts usually. And you can have it run SAMBA services for file sharing.

I know an old Celeron 433 of mine worked well in this role using a Mandrake Linux distro, but if you run something like PFsense, you might be ok since it installs w/o a GUI and managed from a web interface. The more RAM the better though.

You can buy a new router on sale at newegg for $20 shipped or less sometimes, why bother doing that with a PPro.

I use Win2k and some old CAD cards on mine, or just use it as a vintage workstation.

Unknown_K wrote:

You can buy a new router on sale at newegg for $20 shipped or less sometimes, why bother doing that with a PPro.

I use Win2k and some old CAD cards on mine, or just use it as a vintage workstation.

[TIN FOIL HAT] Because the government has a backdoor in all these commercial units. [/ tin foil hat]

But seriously, the features of a home built unit make it worth it.