When it comes to the type of design work that most sign shops do, there's not a whole lot of demand on the actual video card, and most of it is based on the CPU performance. As others mentioned, the memory on the card is no longer the largest deciding factor. But for any design work, 512 MB should even be plenty for anything you really need, much less two 512 MB cards. Using just the one would be ample power and remove some complexities that your software might not even be able to take advantage of!
As Sabre mentioned above, there are several that like the Quadro line from nVidia. I am one of them that will suggest this line of video cards because they have been outstanding quality for me. A simple nVidia Quadro FX570 is what I have used in several of our design computers and it's more than enough power. But even then it may be more than you really need, as spending up to $100.00 will still net you a very decent card for higher resolutions and plenty of performance.