Queue said:
Are we talking Slotket (sp) adapters here?
Queue
It depends... My motherboard, for instance, is Socket370. His might be Slot 1. I THINK that if you want to use a Pentium III in a Pentium II motherboard, you can't use a slotted PIII because of voltage issues - you have to go with the slotket.
The motherboard has to support a 100Mhz FSB, though. Mine does for some reason - maybe consideration for future Celerons (even though my mobo can't support a Coppermine Celeron without a BIOS upgrade).
And it depends on the chipset of the motherboard. ZX is really easy to upgrade, for instance.
So, if you must have a faster CPU, and you must have it on the cheap, it can be done. The only reason why I don't necessarily lean toward recommending that route, is that a computer with an old CPU will most likely have an old hard drive, an old video card, etc.

If the CPU is constantly bottlenecked by everything else in the system, then it's not going to be as much of a speed boost.
Plus, there are other considerations with an older motherboard, that a BIOS upgrade can't address. I recently bought a GeForce2 GTS video card, as an upgrade from my Voodoo3. I found out that my motherboard can't supply adequate voltage to the GeForce, so I can't use the card. It's currently gathering dust on a shelf until I can afford the other parts I need.
I can dig up more detailed instructions on how to do this, though, if anyone wants.