Upgrade Old Computer

If anything, you should upgrade the memory and the hard drive. AMD and Pentium processors are not compatible. They each have their specific motherboard requirements.

for the money involved, buy a new one

You can find a used 2.4GHz computer for about $400 in the newspaper classifieds.
An entire system, about 15 times faster than your current one.

Other than that, adding memory is usually the biggest upgrade -- but only for people who run many programs at once, or edit large documents. To see if this is an issue in your case, run your normal programs and hit Ctrl-Alt-Delete, press the button for "Task Manager" and the tab for "Performance".

It should tell you how much "Physical Memory" you have and how much is currently being used. If your programs require more memory than you have then the system starts using the hard disk instead of RAM, making the system slow way, way down. But if you run all your normal programs and still only use half the memory in the system, adding more memory won't speed things up at all.

Depends on what you want a PC to do for you:
1) Browse the web? Nothing you do will speed up your modem.
2) Edit a short document? It's quick on any system.
3) PhotoShop many large photos? Get lots of RAM and lots of disk.
4) Burn CDs? Nothing you do will speed up your burner.

5) Lots of reading and writing? A larger 19-inch monitor will double your productivity, and the new LCD flat-screen monitors are much easier on your eyes, letting you work longer hours before you turn into a twitching zombie. Yay!

6) Keep your system clean and organized? Don't install dozens of random programs from the web. Many of them consume memory with silly tray tools, toolbars, and "convenient" background programs. And run an anti-virus program often, or your system will slow way down with all the crud it builds up.

I vote for buying a new one. There is nothing you can do to speed that computer anywhere near what you want. A 400Mhz processor is not compatible with that motherboard. (400MHz is a PII... yours is a PI) And with a bus and RAM speed as low as that, it wouldn't make much difference anyway. Just get a new one, you'll be WAY happier... and spend about the same amount of money.

:O)