OK...? I hope everything is cleared up.
Use the ATX case you have along with the 500W PSU.
The Dell case is a mATX case, as is most cases used for prebuilts. They will only hold a microATX board. In this case, since it will be used for office type stuff, an ATX board is not a necessity and a mATX will do just fine. Dell used to use all proprietary parts, so using the Dell case could cause problems if it's layout is proprietary.
Use the ATX case you have along with the 500W PSU.
The Dell case is a mATX case, as is most cases used for prebuilts. They will only hold a microATX board. In this case, since it will be used for office type stuff, an ATX board is not a necessity and a mATX will do just fine. Dell used to use all proprietary parts, so using the Dell case could cause problems if it's layout is proprietary.