yup if your gfx card is a 7950gx2.. then i think your out of luck using the computer to route the signal - if it were one of the newer ATI all in wonder cards then i think that would have the connections needed - You could try a TV card, but im not sure exactly how this would work - you would need to fake it into thinking your xbox was a tv channel..
It looks like your out of luck for this one, unless you find an upscaling box which can do the dells Full res (which im guessing will be highly unlikley)
If you have a projector which can accept the 720p then i think thats your best options.. as it will be considerably bigger than the 30 lcd anyway.
Sorry couldnt be of more help, maybe someone else might have a suggestion.
Cheers
ROfu