All PC browsers

Fist of all by a Mac - then you can test the Mac browsers fairly easily (and now you own a Mac and be happy), I'm not going to cover testing in multiple versions of Mac browsers here as they are much less of an issue than the IE problems that we all love.

Second get Parallels and a copy of Windows (XP SP2 and above i believe). This is a great bit of software that's come on leaps and bounds over the versions. The latest version, 5 at the time of writing, is very fast and efficient. I'm currently running 3 versions of IE in Parallels, when idle they no CPU at all. Parallels itself uses only 6% or so when idle. This is a great leap forward from the last version.

Third go here: http://spoon.net/browsers/, this enables you to test all the PC browsers that you'll probably need to test on and more. It uses some great software that allows you to run any program over the web, you're basically looking at a browser running on a machine on their servers, but you'd be pushed to tell.

There is another way of running all the version of ie, you can install them all. Though i've found it slightly buggy and you don't get the best versions of those browsers always. But here's how you do that if you want to.

I believe that you'll need to be running XP for a start, any higher and you might struggle to run the older versions, but don't take my word on that.
First of all upgrade your version of IE to version 8, now you've got that one
Get IE 7 here: http://tredosoft.com/IE7_standalone - download the installer not the program only version.
Go here to get versions 3, 4, 5, 6 of IE: http://tredosoft.com/Multiple_IE

Once you've done all that you can see a pile of evil in your taskbar (every version of IE stacked like a pile of nastyness on top of each other. Urrrgh (cold shiver))