There's difference betw. GeForce2 GTS and Pro.
GeForce2 GTS 32mb vs. 64mb: almost no difference and 32mb will par or outperform 64meger upto 1024x768x32 bit in many games I belive. So yes, that extra 32meg will hardly be used if the graphics setting/intensity is not asking enough. You'll start to see using up the rest of that extra 32mb when you get into higher resolution and/or intensity of what's happening in the game but you'll not see much of performance advantage.
GeForce2 Pro which only available in 64mb and this one does have considerably higher memory bandwidth so it does have quite a bit of improvement in running heavy games over GTS 32/64mb. Pro is way to go. It tries to compete with the GeForce2 Ultra sometimes.