I would go with a Strict DTD. Read this article, it should help you a lot and it's short DTD
That won't guarantee cross-browser compliance, but it will help. There are many pitfalls related to positioning that will render different in IE and FF depending on the version you use. Read up on IE vs FF with regards to absolute, inline, etc positioning. Once you get your page structured correctly in multiple browsers then you can concentrate on how it actually looks. Post some links or CSS if you have a specific area you can't seem to get corrected.