Don't even think about putting your Dell in a new case. Ultimately I don't think it's worth the hassle. Dell screw everything so tight and fit everything so that it fits their case. And I don't think Dell would repair your pc if it came in a Thermaltake...
You don't need a new case although you could do with some airflow. Take a look at the Zalman VF900 or VF700 to cool your videocard (the latter being cheaper but older).
yea, the way dells are made, the only cases that a dell mobo will fit in is another dell. if you want to get a new case, just remember, with a new case you need a motherboard, new power supply, and heatsink for the cpu.