Honestly, I highly recommend you to get your own
custom upgrade, especially if you care about the original Mac OS.
It will surpass
any "main market" 3rd party upgrade like Sonnet's or PowerLogix'/newertechnology's, as it's the only way you can run the best 32-bit PowerPC CPU (7448/7457, be it single or dual) on the best Mac machine capable of running those systems (MDD, and even the Xserve G4s, which share the same type of CPU module), with also the additional benefit of being able to engage in
silicon lottery (if you so desire) to push for ever-higher stable clock speeds than any CPU upgrade so far could offer.
And it's nearly-always
cheaper than buying the used, inferior upgrades from those companies. And that too assuming you
do find them, because regardless of location, they have become
insanely rare to come by (Sonnet's 7447 MDX for MDDs and newertechnology's 7448 MAXPower for post-Yikes pre-MDD Macs).
And doing this is a lot simpler than it may initially sound like. It's absolutely doable, and I say that with a dual 7448 @2.0GHz coming my way as we speak:
To do this, you need someone who can solder chips (CPUs etc.) and can provide an interposer board for 744x chips to be used on the stock CPU board, which expects 745x chips, as the two subfamilies have different pinout due to L3 cache. For a 7457 upgrade, the upgrade is even simpler, as no interposer board is required on the stock MDD CPU (you can even upgrade the L3 cache speed to be faster with a faster chip the same way, if you want to go the extra length).
After acquiring that, you can use a firmware patcher like Powerlogix' as with the usual upgrades. If using any of the 3 stock heatsinks, use copper shim(s) for the 7457 to take into account its slightly lower height compared to the stock 7455, or, in the case of the 7448 that has additional height due to the interposer board, use washers/spacers between the posts and heatsink to raise the height at which the heatsink stands.
And voilá, enjoy the Power(PC)!
You can find 7448/7457 chips to hand over to your service provider (assuming you won't service it yourself) with patient searching. A lot of Chinese vendors sell them cheap, usually desoldered from another unrelated machine, in the maximum base configurations (1.267GHz for the 7457, 1.7GHz for the 7448, the highest official binning values Freescale was willing to bin -- they can be even higher in reality). The 7448 is still produced and available brand-new, as well, but not for less than hundreds of dollars (400~600-ish), so I don't quite recommend that route.
We are currently discussing this approach
here, in case there are further questions.
