You could try one of the highest rated 500-600w power supplies on newegg: http://www.newegg.com/Product/Product.aspx?Item=N82E16817371016
It's also the same color as the original PSU in your Dell ATM if that matters to you =D. It's a good idea to get a little over the recommended wattage of a video card because it gives you head space for upgrades you might want down the road. The PSU I linked isn't the best on the market by far, but it's inexpensive, and it also has an 80 plus certification for efficiency, although now there are higher 80 plus such as 80 plus gold and platinum but when looking for a cheap simple PSU upgrade this should work fine. Read the reviews of the product to be sure it's what you need.
And one other thing, whenever you get the money to do so, I highly recommend you build your own rig. People probably tell you this all the time. But it's because pre-made computers are way over priced(especially HP, Dell, and other major brands) for the performance you get. All to often they claim to be great, and then less than tech savvy people who want a gaming PC get one of them and realize it wasn't such a great buy. It's the integrated graphics that is the problem gamer's have with them, other than the price.
One last thing, (this is a little off topic) a brand like "Alienware" do in fact have good gaming computers, but they are even more over priced than a Dell or HP.
The more you know!