Maybe I'm not connecting the dots on my own, but I'm guessing based on the info in your post that 512KB L2 can cache up to 64MB of RAM? Still, I'm not understanding how bumping RAM up to 128MB turns write-back cache into write-through cache. I guess I would've thought that if you go >64MB than either the cache only works for data in the first 64MB, or it doesn't work at all. What you're saying is the cache will work for any data in the entire 128MB, but that it reduces performance from WB mode to WT mode?

Well, i know that the VIA chipset can cache the ram up to 64Mb, when 512 kb L2 is onboard.

As i already have told, there is this option "Sustained 3T Write" in the BIOS.
I thought that with "enabled" this function, it can be cacheable (WT) up to 128Mb.
Obviously this option is not visible in my BIOS. But in the manual it is described and visible on the picture.

Then, i have just run the benchmarks, first with 64Mb, second with 128Mb. To see what is the effect on it.

WB Caching area is limited by the Tag RAM and not by the Cache RAM.

On a 486DX2-66 WB is actually a tiny bit slower than WT as WB adds another cycle on write.
On newer systems this is better buffered and due to the higher clock speed ratio between RAM and Cache WB gains the lead.