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.
