I would say that you should only expect users to have a monitor with 800x600 resolution. The Canadian government has standards that list this as one of the requirements. Although that may seem low to somebody with a fancy widescreen monitor, remember that not everybody has a nice monitor. I still know a lot of people running at 1024x768. And it's not at all uncommon to run into someone who's running in 800x600, especially in cheap web cafes while travelling. Also, it's nice to have to make your browser window full screen if you don't want to. You can make the site wider on wider monitors, but don't make the user scroll horizontally. Ever. Another advantage of supporting lower resolutions is that your site will work on mobile phones and Nintendo Wii's a lot easier.
A note on your at least 1280 wide, I would have to say that's way overboard. Most 17 and even my 19 inch non widescreen monitors only support a maximum resolution of 1280x1024. And the 14 inch widescreen laptop I'm typing on right now is only 1280 pixels across. I would say that the largest minimum resolution you should strive for is 1024x768, but 800x600 would be ideal.
