When discussing browser testing, aside from a few isolated differences, the browsers render pages identically except
for Internet Explorer (any version). The adage "Code it for Firefox, hack it for IE," certainly applies. It sounds like you're doing well in designing a site that looks correct in standards-compliant browsers and then wanting to ensure it looks fine on "the" non-compliant one.
That said, your questions:
1. Up to you. Most folks here (myself included) don't feel the need to go back any farther than IE6. I test in Safari, FF3, IE7 and IE6 and occasionally Opera.
2. Browsershots is good. Many people have recommended IETester
3. Here's an article
that might interest you as well.
4. Some form of "My site doesn't look right in IE" is probably the #1 issue that comes up on this particular forum, so... pick a thread, any thread.
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