There's a certain amount of video memory required to play a certain game at certain settings. For example, if you run games like SS2 or COD2 at 1600x1200 with AA and AF, then 512mb video mem is required for optimal performance. Even if you had a x1900xtx, but it only had 128mb of video mem, the performance would degrade significantly. Once you have enough vram, faster ram can increase performance, but it will not make up for the difference if you dont have enough vram.
On the other hand, a card like the x300 is limited by the slow gpu, and if you play any game at settings that require 256mb or 512mb, the performance will suffer from the slow gpu, regardless of how much vram you actually have, and having faster vram will not make up for the slow gpu either.