testing my size in different resolutions

Could somebody please confirm to me which website will provide accurate simulations of 800x600, 1024x768... because I have two and they both show the pages at different sizes. I am using a MacBook with a resolution of 1280x800:

1. This site seems to show everything a lot smaller for me:
http://www.websitesthatfly.com/services ... n-test.htm

2. This site I believe shows it accurate for me:
http://www.webconfs.com/web-page-screen-resolution.php


Can anybody confirm either one or if there is one that come from the w3c for example or one that definitely works. Number 1 does disclaim that I will need to be using IE so maybe thats the problem, I am using a combination of FireFox 3 and Safari 3 (latest).

Thanks

You could look into https://addons.mozilla.org/en-US/firefox/addon/5792

Thanks very much.