The accepted "standard" in desktop resolution is currently set at 1024x768 for any system newer than 2000. Some new systems (actually a majority) are now shipping with standard resolutions higher than that, but many home users are still running in the 1024 range.

This is actually due to two main differences in "modern" systems. The first is the expense and availability of larger monitors (its very uncommon now to see a desktop with smaller than a 17" monitor) and the second is newer OS's. OS's like Vista, 7, and most Linux (I'll keep Mac out for now) distros choose to have their native resolution at a minimum of 1024x... Some OS's (like 7) can detect multi-screen or wide screen setups and will adjust accordingly.

Remember, you can always look to the statistics (example: http://www.w3schools.com/browsers/browsers_display.asp) to tell you what the "average" user is using.

Going forward its more likely that we will see 1280 take over the default area, but first LCD's with a native resolution of 1280 will need to become common.

As AOD said, if your building in 2D none of this is a problem for you. If your running in full screen, then find the device caps that comes closest to what you expect and black fill the areas (either above and below or left and right) that you arn't going to use. If your running windowed just lock your screen size and call it done.

Of course, you can also do things like image scaling, show more of the world, use the empty space as ad space, etc, etc, etc...

- Jeremy