It still can be used as a NAS Server - good for backups.
I have a similar build as my HTPC - just added an eSata 1TB drive and 2GB RAM (4GB total - only recognizes 3.3GB). It does have issues recording 3 HD shows at once, but functions fine with 2.
I'm just worried about its power consumption from being on 24/7. IIRC Pentium 4 wasn't spectacular in terms of efficiency. If it wasn't affixed to a proprietary motherboard I would have tried underclocking it.
If you're curious about Linux, there's a good use for it! You might really enjoy seeing what Linux has to offer, all the different visual interfaces etc.
Another machine already fulfilled this role as an Ubuntu test bench. But I certainly would look at another distro if I decide to make it into an NAS backup server.