My suggestion? Keep it for a command-line only application, router/firewall/NAT, or a dedicated SETI@HOME box if you can, but in terms of upgrading without changing the motherboard I'm thinking it isn't worth it. Sure, you MIGHT be able to incrementally change the processor speed or ram, perhaps, but it wouldn't be worth it because it would still be quite obsolete.

It makes me think back to the day...

You can put together a high quality box nowadays for less than $700, actually. I'm currently looking at a list of dream parts, and here are some of the components from my spreadsheet and an estimated cost total:
  1. AMD XP 1800 (1.8 ghz equiv, supposedly)
  2. 512 MB of PC2700 memory (takes advantage of 333mhz board)
  3. ASUS A7V333 (Onboard simple audio, firewire, USB 2.0, AGP Pro slot, 5 PCI, 3 ram slots)
  4. Antec Performance Plus 1080 (black) Case (w/ 430w power supply, 1 firewire 2 usb ports on front panel. Removable intake filter. Drive door. Etc etc etc.)
  5. EIDE 60 GB Hard drive
  6. 1.44 Floppy
  7. CD Burner/CD Reader (32x/12x/48x)

Estimated total cost without taking into account motherboard shipping and time of assembly...
$571.00 . As a sheer guess, actual costs may vary upwards to more like $600-$630.

Items missing from this list that you would probably want or changes to make:

The case is a bit overkill. You can find much cheaper beige boxes.
You'll need a monitor, Keyboard, Mouse.
CPU cooling device