There are many utils available for watching your website in different resolutions.
Some are installed on PC, some are online services like these:
Adobes "render browser" - you'll need to create a free Adobe account:
http://browserlab.adobe.com
Nice, but not free if you want it fast as I recall it:
http://browsershots.org/
Google - shows what part of your website is visible based on statistics from Google:
http://browsersize.googlelabs.com/
But as far as I know, todays most commonly design resolution is 1024x768, eventhough the height isnt that important because of the popular scrollwheel on most mice/pads.
If you design to design for this resolution there is a lot of design help in the "growing standard" called 960 grid, which is based upon how many professional designers build a design.
You always make an invisible "grid" and then you use the cells to arrange the contents. Much like the old "table" system, but much more focused on professional designs.
Regarding your question, the % you list, seems like you would have most use of the "Google" link above.
Lastly, always remember to test how your website looks with the most used browsers. For me those 4 will be:
Internet Explorer (latest and previous version, use more than one computer to test if you cant do a double installation of it)
Mozilla Firefox (latest and perhaps previous.. but they make less changes in the render, so latest would be ok)
Google Chrome (awesome debugging tools too, and a very stable browser)
Safari (so you make sure that you support MacOSX)
Thats my opinion.