What's the best resolution to design for?

very few people will be using vista for a long time yet.  I personally have no interest in it.

800x600 will remain a standard resolution for years to come.  You may be running 1280x1024 (personally im running triple 1280x1024) but a lot of professionals still use/design for 800x600. 

You should also ensure your web sites conform with this resolution less you risk being prosecuted under Anti Disability Guidelines. Its not a direct offense, but by building larger than that your making things less usable for disabled people.

You may have 0.98% of peopl eusing 800x600 but that doesn't say anything if you only got a total of 5 visitors a month does it.