Dimes to dollars the new mobo won't even fit. Also, the dell 90's (some of them, we have the optiplex here do have the atc type connector on the power supply, but they also use an upboard - meaning a separate board where the pci/isa cards go. This means if you can actually fit a new mobo in you'll have no where for the cards to anchor...
Get a new case... pccost.com has a great one for $90 - multicolored even
Or check out the local computer stores (not the big chains). I found a very nice case for $60 - has sleep button as well, which has been useful. Also came with a fan in the front and room for another in the back. I stole a fan from an old powersupply to put in the back and it works great...