Hi Pagetta!
Most likely your problem is this: the chosen DTD throws IE6 into quirks mode, and so it uses the old "broken box model" of IE 5, and the pages get rendered different.
Information about doctypes can be found at "Activating the Right Layout Mode Using the Doctype Declaration"
http://hsivonen.iki.fi/doctype/
Information about IE's broken box model:
http://glish.com/css/hacks.asp
Note that the hack will most likely work with IE5, with IE6 you *need* a proper DTD.
As for testing with other browsers: I recommend - as always - "code for firefox, debug for ie". Since IE has the worst support of CSS2 of all browsers, it is more easy to write for compliant browsers (ie. mozille, ff et al) and the get it tweaked for the ONE non-compliant browser. Usually, clean validated code which displays well in ff/mozilla requires no or very few adaptation for IE. The other way round it is just more complicated: You have to force compliant rendering engines to adapt to the quirks of IE ...
hth,
Alex
BTW: If you are going to use FF, get the developer toolbar ... it is an invaluable tool.