We've been through a lot of different games, settings, and data. What's the bottom line? How much graphics RAM is the right amount? We've seen enough to formulate some general guidelines.
For the gamer, there are three main factors that have the most influence on how much graphics RAM you'll need: resolution, visual quality detail settings, and AA. For the most part, 512MB of RAM seems sufficient to push one of these factors to the limit, and in most cases, it can handle two of them at once. But if you plan to maximize all three--the highest resolutions, visual quality settings, and AA--then more video RAM than 512MB is a good idea.
The primary consideration should probably be resolution, because this is a hardware limitation for many of us. Typically, 20" or smaller monitors will cap out at 1680x1050, so in this case, spending extra on 1GB of graphics RAM might be frivolous. Conversely, if your monitor is 21" or larger with a 1920x1200 native resolution, purchasing a graphics card with 1GB of RAM might be a wise investment for only a little extra money.
A 2GB card is probably only realistically useful for folks who push 1920x1200 resolutions and above, and who demand the highest visual quality settings in conjunction with AA. Since the price difference is substantial, a graphics card with 2GB of RAM is probably overkill for anyone who has a monitor with a native resolution lower than 1920x1200.
There are other factors to consider, too. Longevity might be a concern as games will inevitably require more and more graphics RAM as time goes on. More importantly, do not forget that the amount of RAM on a card does not indicate its relative performance--a 512MB Radeon HD 4870 or 896MB GeForce GTX 260 will always be faster than a 2GB Radeon HD 4670 or a 2GB GeForce 9600 GTS. The point is that the type of card is far more important than the amount of RAM onboard when it comes to raw performance. For a general idea about what cards are faster than others, take a look at our video card hierarchy chart.
In the final analysis, even 512MB of graphics RAM can provide excellent high-resolution service as long as the user is aware that lowering quality settings and AA will allow the card to perform at its peak potential. However, if you plan to use the highest visual quality settings in addition to AA, it's likely worth the extra money for a 1GB or even 2GB card.