Does Dell make their own mobo's? If not you can just try upgrading the bios.
It could be that this is just a compatiblity problem. The 8800 GT is a PCIe 2.0 card and I've seen some people having problems when inserting a PCIe 2.0 card in a PCIe 1.0 16x slot. So the compatibility might be even worse when putting a card like that in a PCIe 8x slot. I'm not sure, it's only a suggestion.
If a 8800 GT doesn't work, I don't see a reason why a 9800 GT would.
If it is a problem due to your bios or chipset, it's just a game we're playing. Every card you try is just a gamble. Some may work, others might not.
And if I'm allowed to gamble on this forum, I would say the card I suggested might work. It's from the same series as the one that did work, only this one is faster. Another gamble: an 'older' card from the 8800 series might work too (I mean the ones based on the G80 GPU like the Ultra and the GTX)