Dell's are not the easiest PC's to upgrade a psu in, I had to cut sheet metal tabs out to fit one in my old 8400, but it can be done. As far as a gpu goes, it kinda depends on the CPU. Anyone can throw a 780ti in a PCIe slot, but its money really wasted if paired with an i3 2120. So, what cpu?
Not all 500w psu's are exactly that. Some I'd be happy to pair with a 760 or 270X, some I wouldn't pair with a 640 or 240X. So what psu?