eVGA 7900GT problems in Dell 8400

DOA card, more then likely.

From Dell....possible.

Your PSU, (although doesnt seem to be the case) make sure its
sufficient for your machine. As I have first hand experience with
some of the components Dell likes to use....  

If your PS can't handle the added drain, it will fail to boot. Happened to me when I installed a 5950 Ultra a few years back and I had the Antec True 380. I upgraded to the Antec True 550, no problems. I have never received a dead video card but I'm sure it can happen. I have noticed some cards will have a thicker flange that keeps the card from inserting in the PCIe/PCI all the way, it gets hung up in the guide slot beside the MB. Double check to see if it is fully inserted in the PCIe slot. Also look to see if one of the wires on the molex connector have pushed out.

Considering all his other cards work/have worked... and the currently purchased uses no more, or minimally more power... still inclined a DOA.

I'm a VAR, and I have seen way to much DOA, near term DATS lately, too many infact. I use all first and minimally second tier manufacturers too.

As a system builder and I have yet to see a DOA video card. It isn't impossible though, not by far. Static electricity kills a lot of components. What normally come to me as DOA cards usually ends up something odd but easily fixed. It may very well be dead but look for other things first.