Which FPU?

[Piru]

@MrZammler

Does the FPU need to run at the same speed as the CPU?

No, it doesn't. You can run FPU slower frequency than CPU.

However, with Blizzard 1230-IV you can't adjust the frequency of the FPU independent, so you need to use 50MHz part.

Blizzard 1230-III was able to run FPU on separate clock, so you could use 25, 33, 40 MHz 6888x. (However IIRC, Blizzard 1330-III manual suggests you don't run FPU at lower speed than CPU).

Will a PGA e.g. 33 Mhz FPU work?

In general, it would, if the card would be able to clock the FPU separately. But 1230-IV can't, so you need to use 50MHz part.

[Piru]

@DoomMaster

YOU ARE WRONG !!!!

Do your homework

Read MC68882 Technical Summary (BR509), page 5, section "BUS INTERFACE UNIT", last paragraph.

Reproduced here, for your convenience:

Since the bus is asynchronous, the MC68882 need not run at the same clock speed as the main processor. Total system performance may therefore be customized. For a given CPU performance requirement, the floating-point performance can be selected to meet particular price/performance specification, running the MC68882 at slower (or faster) clock speeds than the MPU clock.

(emphasis mine)