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Personally I stopped worrying about anything less than Netscape 7.0. If a user has that and wants to use my site I flat out tell them to upgrade or they can't use my site.
And if it does not display - your code is not valid, if your code is not valid - it may not work in other browsers either. I am sure you would be horrified if it did not display in version 5 or IE, but you can't test it in that because you have 5.5 and 6 on your machines.
Repeated from Item 5 above:
HTML: [validator.w3.org...]
CSS: [jigsaw.w3.org...]