I looked at Dell's specs on this computer on Dell's website. Yes, this does look like an ATX motherboard, but I don't know whether Dell has made proprietary changes to to it; like mounting screws location, and things like power switch, reset switch, etc.
And the case looks weird! If you are absolutely decided on changing the case, try any Mini-tower case found at your local computer store. Keep in mind 2 things: 1) it is practically like building a new computer with existing available parts, and 2) you will still have a GX-280 Pentium 4, which is now sold for about $132 - http://www.google.com/products/catalog?q=Dell+OptiPlex+GX280&hl=en&rlz=1G1GGLQ_ENUS359&prmd=ivns&resnum=5&biw=1399&bih=770&um=1&ie=UTF-8&cid=5619376994862575570&ei=mt5OTcfBLYfMgQe_2Yg2&sa=X&oi=product_catalog_result&ct=result&resnum=2&ved=0CG8Q8wIwAQ#
It does not seem economically feasible!