High resolution textures can take up lots of VRAM. Some games (Ghost Recon: Advanced Warfighter 2 for example) won't even let you select "high" textures unless you have 512MB of VRAM.
Also keep in mind that low resolutions such as 1152X864 will benefit greatly from anti-aliasing, which requires a lot of VRAM.
At this point I wouldn't recommend a video card with less than 512MB for gaming. Games are only going to get more demanding, and 512MB cards aren't very much more expensive than their 256MB counterparts.