If you want to upgrade to a G3 or G4 there are a few things to consider.First anything over 500 MHz is wasted.
Carrier cards and Zifs are the way to go
Carrier cards are down to about 50 USD
A G3 233 can be had for about 10 USD
Total cost 60 USD, you can swap in a newer ZIF later and sell the old one. Carrier cards allow for easy (simple as flipping switches) overclocking.
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LC 2, LC 3, Q605, Perf 638, Colour Classic (160 603e) 6100, 7200, PTP 225 (Quad 604), PM 9600, G4 Cube