find the part number or psu model and search for its real capabilities.
if its under 450 watts i would not recommend it. your i5 would already be pushing your psu's limits.
If I were you, I'd just spend the $60 to $80 for a solid power supply and get any card I want. It's not like you are looking at a budget card with the 7850. It's worth it to know you have a psu that will not only last for years, but keep all the components in your system safe. swapping out the psu is very easy and safe. It just might take some time to maneuver the cables that Dell might have zip tied or hid inside the case.