Nah - Dell uses good PSUs. The stock PSU will run any HD 5670/6570/6670/7750. Too bad you will need a low-profile card. Those are harder to come by.
Luckily this one is on sale currently, and it will do wonders for your gaming experience.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102958
Here's a review from someone using it to play Diablo 3 with a 220w Gateway PSU:
"Pros: diablo 3 highest settings, working fine (so far) with stock 220w psu, not loud.
Other Thoughts: This was my first DIY computer upgrade attempt and it went rather smoothly as everthing is still in working order and Diablo III is being played on higest settings: the reason for my purchase.
Was concerned that my 220w psu is not even close to the suggested 400w psu the manufacturer recommends for running this card. After reading all the reviews I decided to risk it and after about 10 hours of gaming, so far, so good running on the 220w psu..."