I have a Dell Dimension 8400 -- this is in their pretty standard mid-tower ATX case. These cases change slightly over time with new models but don't change much ... so it has the one heat-shrouded exhaust for the large CPU fan.
Recently I've had the opportunity to test two separate, but identical, 7900 GTs in this machine. Before shipping off one of these to a friend, I first benchmarked the two cards for some days in my Dimension 8400.
For both of the 7900GTs, the temps were idle 48 C and hot 61 C, in my Dell.
I sent off one of the cards to my friend, who overclocked his from 450MHz to 562MHz ... and his temps never rise above 55 C even when running 3dMark06 for thirty minutes.
So, it pretty much confirms what I had suspected long ago, which is that Dell has particularly poor cooling solutions in their mid-tower cases.
I'd like tips on what I can do to help cool my machine. It might be that you would need to be familiar with Dell's cases in order to offer the best advice.
Actually if I were going to provide air in the front of the case, I would just insert a 5.25" bay fan into my empty spot.
For now all seems good, though. I have a PCIe 7900 GT in there and have overclocked it more than 100 MHz on the core. However I put a Zalman VF900 Cu on the graphics card, lowering the core temps (stressed max) from 69 C to 48 C.
A few things about the older Dell cases. They are steel covered in plastic. Makes it particularly difficult to cut holes and such. It has a thick layered effect. One thing you could do is install a slot cooler that sits under your card and exhausts out the back.
I did mount a front fan on mine, since there was a sort of grill there for air intake (I used zip ties). One thing I thought about doing was using some cardboard to funnel the air directly to the graphics card and leaving some slots open to exhaust the air. You can make a cheapy air filter from a used dryer sheet.