Another SX part can be put there but not a DX due to completely different pinout and package.

After that amount of effort an slc2 would be better

Also, keep on mind that you may be throwing your ISA bus out of specs (unless it is clocked by another oscillator or YOUR BIOS can choose the bus speed).

null

No ISA bus runs at 20 MHz. There's either a fixed divider (based on CPU clock) or it's set in the BIOS. For 20 MHz, it's normally 1/2, so ISA runs at 10 MHz. For 25 MHz, it must be 1/3, so that the ISA bus runs at 8.33 MHz. If you leave it or can't change it, the ISA bus runs at 12.5 MHz, which almost no ISA card supports. 8.33 MHz is the standardized speed, 10 MHz is tolerated by most cards.

12MHz was about the limit for that 16-Bit Bus. Ironically, older AT-Bus cards from 86-90 or so might be more tolerant to high speeds.
But yes, the standardized 16-Bit card type we now know as "ISA" was frozen at ~8MHz, though. 😀

derSammler wrote:

No ISA bus runs at 20 MHz. There's either a fixed divider (based on CPU clock) or it's set in the BIOS. For 20 MHz, it's normally 1/2, so ISA runs at 10 MHz. For 25 MHz, it must be 1/3, so that the ISA bus runs at 8.33 MHz. If you leave it or can't change it, the ISA bus runs at 12.5 MHz, which almost no ISA card supports. 8.33 MHz is the standardized speed, 10 MHz is tolerated by most cards.

That's what I mean. The ISA bus speed was usually taken from the main clock (using a divider), so changing the main clock will accelerate the bus (if your board used main oscillator for ISA bus, your cards are running 25% faster than before due to the change from 20 to 25 Mhz). On some boards, ISA had its own clock so changing main oscillator won't affect ISA bus.

As derSammler said, 10 Mhz is the highest stable speed in most cases and going beyond that wouldn't be advisable. Changing from 20 Mhz to 40 Mhz would mean a 100% overclock, so it's very likely that you computer won't start (thinking about that... are you sure that your mainboard chipset will be capable of such overclocking?).