It also kinda depends also what crowd you're aiming at as well - for example, the computers the casual game crowd have tend to have less vram. A lot of integrated cards use system ram as vram though, so vram isn't really an issue for those even though they're generally not very powerful.
Basically, yeah, 128 is a good amount to shoot for. I don't think it's necessary anymore but 64 is really the minimum you should bother try for if you want to be sure older computers can play it too. Keep aware that other things like the operating system have to use vram at the same time, though so you might not have a full 128mb to use on a 128mb card!