First of all, running a 366 at 458 is a very bad idea, because you're running an FSB of 83 (5.5x83=458), which gives a PCI bus speed of 41.5 (83/2=41.5). Hard drives have a pleasant habit of becoming corrupted at that speed, especially some 2 year old ones. A better speed to run is 75 MHz FSB, which will give you a 412 CPU speed.
With your P2B, you SHOULD be able to run a Celery Coppermine but it depends. Your motherboard revision should be printed on your board between the ISA (or was that PCI?) slots. If it's 1.02 or later you can upgrade the BIOS to 1012 and run a Celeron Coppermine. I would suggest buying a Celeron 566 with an MSI MS-6905 Master slocket, and a good heatsink and fan. Jumper the slocket to 1.8 V, and happy computing. If you're lucky, you'll be able to run that FSB at 100 MHz --> Celeron 850, assuming your memory is PC-100. (The PCI bus will be fine since 100/3=33 MHz (normal)). If you don't want to take the chance you can buy a chip that will be guaranteed to hit 850. Not sure if CompuWiz here still sells those but you should look into it.
Edit:
I see you already have PC-100 RAM.
I would suggest (assuming you have mobo rev. 1.02 or later):
Celeron 566 (guaranteed is better - dunno the price)
Golden Orb and thermal paste (~$20)
MSI MS-6905 Master slocket (~$15)
Another stick of 64 MB RAM (~$35), or even better, another 128.
A better OS if you can find a copy cheap. (ie. Windows 98 or Windows 2000)
OTOH, if all you're doing is surfing and Word, etc., all of these upgrades will be largely irrelevant (aside from the memory upgrade).
