My feeling on it is that no smaller than 800 for basic layout. However pimp images should be as big as they can be to load fast and look good. Rick is spot on with the target audience thing. Just depends on who you're targeting.
If your target audience (remember that one?) is artists you'll have some leeway to use a larger resolution. And remember that the screen resolution isn't always what people run their browsers at - some folks maximise the browser and others don't. I don't on my 1900px mac or my 1600px PC, but do on my EEEPC I look at…