The easiest - and in my view best - way to speed up a Quadra 700 is simply to swap in the motherboard of a Quadra 800. It's the identical form factor and just goes right in.

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Wrong. The Quadras 700 and 800 are quite different. The Q700 case is based on the Mac IICX one (a 030 mac). A Q800 would fit in a Q650, and theoretically a Mac IICX or CI.

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If a Q800 motherboard would fit in a Q650 case then it should also fit in a Q700/IIcx/IIci case. I have a Q700 motherboard in a IIvx case which is identical to the Q650 case, but before that I had it in my IIci case. I had to hack the back panel because some things don't line up exactly, but it did go in.