Therin lies the problem, because Dell use Dell parts, there are no full specifications available for their motherboards (that i know of). I've just tried firing up trusty old Everest, and not even that will give me voltages for the AGP port/AGP version. Short of phoning up Dell or ordering a new motherboard i'm not sure there is a way of telling.
A word of caution though, I tried replacing a Geforce 2MX with a Geforce MX440 without checking voltages before, and the result was a crispy MX440 and the distinct smell of burning.
Sorry I can't be of more help
The best thing to do is google your card and get its AGP version/voltage. Then find a new card with the same.