Why not get a low profile video card? Newegg has a Jaton 3DForce fx5200tv card for $35 plus shipping. It should be similar in performance to your old card; this is cheaper than buying a new case and ps.
All of your components will fit no problem into a standard mid size case with a new PSU. Your biggest issue is the Dell motherboard uses unique connectors for the front panel (pwr, reset... etc) and front USB connector ( http://support.dell.com/support/edocs/systems/opgx270/e... ). You will need to be prepared / capable of slicing wires to make the new case connectors work, if it will even work.