I highly doubt that the cooling would be sufficient for such a big jump... the TDP, not unexpectedly, doubles from 500mhz to 1ghz. I wouldn't try it, personally.

But there's also another issue... have you checked to make sure a new CPU would even fit? The earlier mobile PIIIs usually weren't in a socket, but on an MMC2 daughtercard (for which, anything over 700mhz is quite rare to find).

BTW, what kinda iBook, and what are you gonna do with it? 😎

Hmm, assuming it is socketed - what would be the ideal upper limit?
The manual mentions 650MHz but I'm not sure if that is a design limitation or if faster hardware was not available at the time.

iBook
It's a nice 1.2GHz iBook G4 with 1.25GB of memory, it works fine aside from the dead backlight.
The local repair place wants $120 to fix it and people buying "dead laptops" only offer around $50.

Since I don't have the space or cash to repair it it has been sitting in limbo for a while.

Well, if the thing allows a 650mhz officially, you should be able to get away with a 700 or 750, maybe even up to an 800, but I'd be hesitant to go above that. The 700mhz+ processors didn't come out until a few months after the 650, so it is possible that was the fastest available at the time.

It's a 4W difference between a 650 and an 800, so that shouldn't be a big deal, but the 13W difference between a 650 and a 1ghz would be a bit much, I'd think.
.....

About the iBook, the G4s usually aren't as straightforward as the G3s when it came to backlight issues, but there are common problems I know about, and if you're lucky it's one of these, which you can generally fix yourself without much expense: Sometimes the easy fix works, just resetting the PMU. Not often, but sometimes. If the screen flashes for a split second at boot, then goes black, it's usually the inverter. If you get no light at all, it's usually that the inverter cable has broken, and sometimes even shorted out on the hinge assembly and blown a little fuse on the mainboard.

I do too think it's likely that this is a CPU module and not a socketed chip. I had a Dell monster earlier this year and it had a 450 MHz Coppermine module.

Manual says MBGA2 package for whatever it's worth.

Well, BGA implies it's soldered on. The question is what it's soldered to... if it's directly attached to the motherboard or some proprietary module, you're outta luck, but if it's on an MMC2 card, then you still have some options. Unfortunately those options would be rather limited, unless you wanna pay the ridiculous sum of money the upper-end MMC cards go for.

I'd say the best option is to stick in as much RAM as you can, maybe a better hard drive if it's still got the original, then just don't plan on doing anything with it that requires more than a 500mhz PIII.

That's okay, I don't need it to be fast - was just an interesting thought.

Sorry for going off the topic, but will it melt ? I highly doubt it, but will it blend ? 😉