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In practice, HTML 4.01 transitional would be a good starting point.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">As Fotiman has said above, that "full" doctype (with an URL pointing to the DTD) changes the rendering mode in modern browsers to a more standards-compliant mode, so if you have problems you can use this instead:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">The above will ensure the more backwards-compatible "quirks mode". Ideally, you should move to the full doctype over time and correct your page layouts to render well in standards-compliance mode.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
a transitional doctype will also tip both IE and Fx into standards compliant mode as well, as long as you've got a system URI. In fact, I'd specifically say not to go for a strict doctype in this case, as if the OP is updating old content it's very unlikely to validate.
Funny you should mention that. The reason I was drawn to this topic was because I had just spent 4 hours trying to get one column to be the same width in IE and FF. Everything else was fine, except this one column. Then I switched to the strict doctype, and *BLAM-O* everything worked perfectly.
