Me and a few friends from work tryed with with a Voodoo 5 AGP. It had 64MB of vRam and we tryed adding 128MB. It worked but Windows didn't let you use more then 64MB. Never got around to build our own driver set. Worked just fine in Linux but really there was no diffrents. Before we tryed DOOM3 as a benchmark before the mod and it was slow as a slug. When we added the memory there was no change. We also tryed it with a few other games with no change in proformance. Thing is it's not worth it, back in the day you were able to add more vRam with the little IC ram chips but then there was a point that it became worthless and just easier and cheaper to buy a new video card.
To live is to let die