It is mainly about keeping textures in memory, newer games will benefit most (like Quake 3 for example).

More info:
http://www.quake2.com/glquake2/news3.html
8Mb vs. 12Mb Voodoo2
John Carmack updated his .plan describing the differences about 8 vs. 12 megabyte Voodoo2 boards. Here it is:

8 mb or 12 mb voodoo 2? An 8mb v2 has 2 mb of texture memory on each TMU. That is not as general as the current 6mb v1 cards that have 4 mb of texture memory on a single TMU. To use the multitexture capability, textures are restricted to being on one or the other TMU (simplifying a bit here). There is some benefit over only having 2 mb of memory, but it isn't double. You will see more texture swapping in quake on an 8mb voodoo 2 than you would on a 6mb voodoo 1. However, the texture swapping is several times faster, so it isn't necessarily all that bad. If you use the 8 bit palettized textures, there will probably not be any noticable speed improvement with a 12 mb voodoo 2 vs an 8 mb one. The situation that would most stress it would be an active deathmatch that had players using every skin. You might see a difference there. A game that uses multitexture and 16 bit textures for everything will stress a 4/2/2 voodoo layout. Several of the Quake engine licensees are using full 16 bit textures, and should perform better on a 4/4/4 card. The differences probably won't show as significant on timedemo numbers, but they will be felt as little one frame hitches here and there.

Is this the same in SLI configurations? I'd like some real game info from real tests. I know that NFS3 didn't display the dashboard on 8MB card as opposed to 12MB card. But would this still apply to SLI with two 8MB cards?

Only memory buffer is extended (allowing for higher resolutions). Texture data has to be duplicated in SLI mode.

Main issue with frame rate tests done back in the day is they don't nearly have as much frame rate analysis as done today. What John Carmack described is a situation with slightly lower average frame rate, but significantly lower 99th percentile frame rate.

SLI is superior for the simple fact that it allows you to game at 1024x768 IMO.

EDIT: Sorry, misleading title I thought you asked SLI 8MB vs single 12MB..

I guess no 1st hand experience/tests then? I'll see if I find the time to do it myself... Although I'd thought someone would already done it 😉

They will stutter. 12MB cards stutter as well, but 8MB cards will stutter a lot more.

The big thing about the difference in the memory is being able to run games at greater resolutions than just 800x600, you can mode for extra memory on the TMUs however it gets complicated beyond just stacking the edo chips as you will have to do a couple of other things but the TMUs according to documentation can take up to 16mb each. The frame buffer (the chip in the middle) is already maxed out at 4mb.

Not sure if the Voodoo 2 is worth it anymore due to the price gouging.

The SLI is overkill for "wanting 1024x768", just get a Banshee or a Voodoo3 at least if you really want that (and more resolutions 😀)

Also be aware there's a few zbuffer-disregarding games that can work on a single Voodoo2 at 1024x768. Also at this res, the filtering and dithering are not applied