Yes. Dell PSU's don't have an over abundance of total wattage, but they drive high amp ratings on the 12 volt rails, THAT is what is most important when considering a mid to high end graphic card. For all practical purpose's, and the element they are designed to work, Dell PSU's are very good PSU's. Your PSU will power most upper mid range GPU's. I would probably go with a 5750, which is a huge step up from what you have. All PCIe cards are backwards compatible, all the way back to PCIe 1.0. No worries there.
The manufactures "suggest" a PSU with much more total wattage, because they have to cover their bases and take into account there are many PSU's sold which do not have strong amp ratings. By the time you get into the 450-500 watt range, even the cheap no name PSU's are usually pushing enough amps to run most any card. (I am not in anyway suggesting to anyone to buy one of them though)
There are PSU's out there that rate 450 watts total power, but may only drive 20amps on the 12 volt rails. When you get into the finer specs of the card, it will give an amp rating as well to go along with that PSU rating. That is the real number you are looking for. So when you are looking at a PSU, you have to do your homework to figure out if it really does fit your needs. The units you see recommended regularly in the forums here will drive enough amps on the 12 volt rails to power any GPU you want to plug in.
Dell isn't going to tell you anything, but what you do want to confirm if you call them is the amps your particular PSU has available on the 12 volt rails, then confirm this is enough for the GPU you select. This is what is important, forget about the total overall wattage.