I personally use PC with Windows 7 (and other slow laptop with Windows Vista). First, I validate documents (using validator.nu, because invalid documents might give bad things). I don’t care about CSS validation, CSS from always had detailed instruction about dealing with invalid documents and CSS3 prefixes cause documents to be invalid CSS.
Next I check sites in newest Opera (my default browser, so I check mostly in it), newest Google Chrome (and as this browser auto updates, I don’t care about old versions at all), newest Firefox and Internet Explorer versions 8 and 9.
I also check pages in Internet Explorer versions 6 and 7, but only to see if random
doesn’t cause site to be unusable. Because I cannot officially have more than one version of IE on PC (and IE6-7 isn’t even available on Windows 7), I use IETester, which is awesome tool to have (along with DebugBar).
I skip Safari because it uses WebKit engine, and the results would be most likely similar to those made by Google Chrome.
The PC with Windows 7 is powerful machine, but that PC with Windows Vista is also useful because it allows me to see how website would be fast on slow PC (where Windows Vista barely works BTW).