First your questions are unclear - look at the title for starters. Isn't what you're wanting to do just the opposite of that (older mobo into new case)? Then you start talking about front panel connections and slip the term USB in there, well which is it? IAC, my answer would be approximately the same. Just reverse them if your intentions are as per the title, rather than the text.
Not all Dell PSU mobo connections are non-standard - I think they've mostly learned their lesson on that one, so you may be a lucky one.
As far as the front-panel wiring is concerned, you just need to know the pinout (what signal each important pin is) of your mobo's front panel connector to use the standard connectors that come with most cases. The hardest thing you'll have to do (perhaps) is shift a contact or two within a shell (usually power LED and/or speaker) - if you don't know how, look in the Help/FAQ section
Mod Header Connectors at Front-X.
Worse to worse, you could remove all the contacts from the factory shell and replace them with the contacts from your new case. That way you would continue the ease of having only one connector to mess with instead of a bunch of tiny ones (I often do that with my cases anyway as I'm in there messing around so much) - or you could always buy a shell of the same number of positions (
http://www.action-electronics.com is a place to look) . But only use the original shell when your intention is to closet the old case - if the plan is to sell it, then leave it as is.
.bh.