Depends on the website - assuming you're asking re: HobsonFiles - I would estimate that you're ok moving to the larger screen sizes due to the likely more technical audience you're building for.

In my own experience planning around 1024x768 in general can be beneficial because:

a) It allows you to better focus your content rather than having space to fill which often leads to overdesign.

b) Often users with higher resolutions are either (a) Older (b) Less sophisticated which depending on your service can be some of your best converting traffic.

c) Interesting note from Nielsen along these lines suggesting that for users < that resolution they will absolutely not scroll to right, so dont put any important content to the right http://www.useit.com/alertbox/horizontal-attention.html