Just thought I'd post this here in case anyone in the future is curious. I have confirmed that you can upgrade the STB Lightspeed 128 to 4MB if you solder on an additional pair of MDRAM chips. The additional memory is recognized by both the card's BIOS and Windows.

My card originally had a pair of MD909 (1.125 MB) chips, and I added a pair of MD908 (1MB) chips. The ET6000 cannot address more than 4MB, so the extra 0.25MB doesn't do anything. It does show that you can mix MD909 and MD908 chips though. The chips are all SJ-5-100. I read elsewhere that you cannot mix different chip types (i.e. SJ-5-120 or AJ-5-100, etc.) though I have not confirmed this myself.

Factory original 2MB - supports 1280x1024 @ 8-bit, 1024x768 @ 16-bit, lower resolutions at 24-bit
Factory original 2.25MB - adds support for 1024x768 @ 24-bit
Upgraded 4MB - adds support for 1280x1024 @ 16-bit

Unfortunately, the card does not support 1280x1024 @ 24-bit, not because 4MB isn't enough, but because the ET6000's integrated 135MHz RAMDAC isn't fast enough.