I don't know if it applies to every notebook, you need to keep in mind that in some cases, the video ram is taken from your system ram.
For example on my laptop, I have ati mobility radeon Xpress 200..., when I set it to use 128mb of ram for the video card, it took 64mb of it from my system ram (If I'm remember correctly, I forget the exact values). So even though I have 1.5Gb of system ram, the video card reserves 128mb, giving me only 1.3Gb of ram.
if your starting off at 256mb of system ram, and the video card takes 64mb of it, that only leaves 197mb of ram for the rest of your computer to use.
windows xp wants at least 64mb, which brings you down to 128 free for every one else.
Whether or not this is true in your case depends on if the manufacturer of the laptop included any dedicated video ram - I think mine came with 32mb of video ram, if I wanted more it would take it from the system ram.
So I would get as much ram for the laptop that a, the laptop can support, and b, that you can afford.