Unless you want to play Doom III with these unnaturally large textures they added for players with 512Mb graphics, then no, there is no known benefit of having more than 256Mb (happens to be the maximum any other game can occupy on highest settings). If you're running multiple applications, that would fall directly into system RAM, not the graphic card's.
Anymahoo, you're designing graphics... so no difference. It would take such a giant collection of polygons and textures in a single instance that even a 128Mb card would respectively be difficult to fill.
For economic purposes, stick with the 256Mb. :)
