Want to upgrade to Lrgr chassis, will dell M.B. work?

I own a dell dimensions 4500s which only has one 5.25 bay ext., and 2 3.5 bays (one int. one ext.) so there is NO room for expansion. I want to buy a new case/chassis for it without having to get a new motherboard, but I talked to some people already and they said that Dell makes their own sizes and it may not just drop into a standard ATX case. The motherboard is ~10x10 inches. It's a p-4 @ 1.8gHz, so I don't really need to get a new motherboard. I want to keep my HD's (one is installed recently, and the old one is just sitting in a box), my cd and floppy drives, all I want is room for expansion. Just wondering if anyone has any info or advice on what I can and/or should do. Thanks.