Actually, render buffer is a quite small portion of the total video memory nowadays. Texture and model data use much more memory, and these are required for model detail. The lowest sensible resolution at this point is 1280x1024, which (including 32-bit color, z-buffer, and double buffer) is only 15MB of memory.
More to the point it is uncommon to buy a 256MB video card since 2007 (and 512MB video cards were readily available in 2006, but most of the people that would buy such a card will upgrade before four years anyway). The game is released in 2010, I think it is not unreasonable to expect a video card 3 years old or newer. Today, it is possible to get a capable video card for cheap, briefly looking around I see ATI 4670 with 1GB which is a quite nice card actually for only $55... and the 4850 which is recommended for SH5 by the devs costs $100, today. You can expect these prices will have come down even more by the time the game is released!
Everyone involved in PC gaming has to just accept that periodically hardware upgrades are required, but by buying an older-model card every year or two, you will benefit from always having adequate hardware for not very much money. You will never have the latest thing but you will always have something that is adequate. (Works for other stuff too... how much is a Core 2 8000-series motherboard and CPU these days?)
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