Here is the target system.

Notice the DIP socket for TAG RAM upgrade below. According to the manual, this is used to extend cacheable RAM from 64 MB to 512MB.

I don't officially need to use it since I only have 64MB RAM. But what happens if I put there a 128K-12ns cache module from a random 430FX board I have here?

Here are the results of different benchmarks:

As you can see from here, there is hardly any advantage in using 12 ns TAG RAM when running the system at 66 Mhz, while at 75 Mhz it gives somewhat better results: 2%-7% improvement in Sandra, 1,6% improvement in Super Pi and no improvement in Speedsys (weird, isn't it?). Topbench results are also better, but there is way too much enthropy to judge.

It turns out that built-in 256-15 TAG RAM can't cope with 75Mhz bus:

1/15ns=66.666Mhz
1/12ns=83.333Mhz

To prove this point, I inserted 256-20 chip (1/20ns=50Mhz). There's a weird 2% performance increase in CPU benchmarks. But it can't handle anything more than 50Mhz bus, so in memory benchmarks it results in very significant performance drop.

Still, it does not affect speedsys results and I can't understand, why.

Conclusion:

Onboard TAG RAM is designed to work only with documented 60 and 66Mhz bus speeds. When it comes to overclocking a descent motherboard to higher bus speeds, it is preferable to upgrade the TAG RAM with the chip that can cope with higher FSB: 12ns for 83Mhz, 10ns for 100Mhz FSB.

If you have an empty DIP socket and a chip to fit into it, you can squeez some extra 1-7% speed from your system.