I have to spread the testing over 2 PCs, one is running XP Pro with IE6, the other is running Vista with IE7, and both have:
Firefox
Safari
Opera
Installed.
Cross browser stuff is not such a big deal to me these days, I've found that (when building my own templates and sites at least) if you take the time to code carefully and test stuff as you go along, you don't even need IE6/7 specific CSS files, or if you do, it only needs a line or two of text.
I'm all out for cross-browser compat, I think it's very important when dealing with paying client's sites...although I can only test on what I have...for example, people running other browsers on Mac or Linux, I think that's taking things a little too far...one can only do so much 
so I ran my site through that validator site & it just says 50 errors.
Ouch 
All errors will give you the exact place where they occur, relative to the output source (in your browser, go View->Source to see what the validator sees) - it's usually pretty easy to erradicate most of the errors...with a handful of tricky ones