Actually, with a 64MB card, you should be just fine. For 32MB cards it is going to be close, and possibly not workable. Looking at the current scenes in Unity, the most VRAM ever in use is about 22MB, and it ranges as low as 10MB. The reason that is disproportionately low is that for animations, it's only usually needing one frame at a time. So while it might be using a ton more system RAM, the VRAM itself is able to offload and onload more dynamically.
As the game is built up more, I expect we'll see a rough doubling of the number of textures onscreen at once, which is what determines VRAM usage. But most of them won't be too huge all at the same time (the 1024px ones really eat up the VRAM, such as for the largest buildings and the ground and sky), since there's only so many of the huge textures that we can even fit on screen at once, heh. Most of our textures are 256px, 128px, or 64px.
In terms of final VRAM usage, at worst that's probably going to put us into the 40MB range, but it might not even be that high. If we do texture-based shadows, some of those will be VERY large and might even get up to 2048px textures, which would use an absolute ton of VRAM. For full shadows, you might need more than a 64MB graphics card to get good performance -- but you might need a better card in general to get good performance with shadows, if we go into them at all. Fortunately, would definitely be optional, and simply could be turned off on older cards.
Ideally I'd like to support even 32MB cards if we can, but that might be tough. We'll see. But 64MB cards, especially with shadows off, should be no problem whatsoever. And even if the game goes over the amount of VRAM you have, I think
that Unity is smart enough to load and unload textures as needed so that it will still run (as opposed to crashing), but that would come at a performance penalty (which matters on graphics cards that are already older in the first place, moreso than on a newer card that would be getting 200fps as a baseline anyway).
So: long story short, you should be ok.