I think you will have to get an OEM one due to it being a Dell. I have never had luck with aftermarket power supplies in them. Here is a link to one:
http://www.etechnext.com/r850g-dell-300-watt-power-supply-for-inspiron-530-531-545-580-mt.-new-pull.html?gclid=CO-H-rn5qMYCFVM7gQodqyENGg
BSA may know better than I.
The physical dementions of a PSU may not fit in a Dell case, but all of the power connectors and power delivery specs are standardized. Ironically the cheap units that dell uses apparently don't deliver power to ATX spec
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story5&reid=154.
The basic message (warning) I want to convey to everyone here:
Don't buy cheap power supplies, or any from a manufacturer without good reviews. Why? Anyone ever had a top end hard drive such as a Western Digital Black go bad within the warranty date? Odds are a cheap power supply came with your machine, sending voltages out of the ATX spec and fried it. That is the best case scenario.
Worst case: A cheap power supply can burn your house down! These generally are the ones that come included in cheap cases, but it is always best to do some research before buying a unit.
Look up PSU models, or at least manufacturers on jonnyguru.com . They not only stress test units, they tear them down and inspect build quality.
Most people treat power supplies as an after thought, but you should still be getting a quality built unit to prevent system crashes, components frying, etc.
Cars are used as an analogy to computers some times, and the power supply is like the size of the gas tank and quality of fuel. These cheap units from unknown manufacturers are like running your diesel engine off of deep fryer oil. As the Mythbusters have shown that you can run a diesel engine easily of deep fryer oil run through a coffee filter. This doesn't mean that it won't kill your engine after prolonged use!
What power supplies are good to get?
Any of the $60 (more or less) 400-600watt units from Corsair or EVGA. There are a lot of other good manufacturers out there, but Corsair has a unit for every type of build, and EVGA is top end with unprecedented warranties. If you see a good deal on something else, look it up on jonnyguru.com first.
I personally recommend as a starting recommendation the EVGA 'Supernova G2' 750 watt units @$130:
http://www.evga.com/Products/ProductList.aspx?type=10&family=Power+Supplies&chipset=750+Watts
"Now hold on....$130????!!!" Yes I hear you. I recommend it because it not only has more value in the long run, it is also theoretically cheaper
in the long run due to it's 10 year warranty. Most $60 units only have 3 year warranties. Now warranty isn't everything, but the build quality of the high end EVGA units are superior to something @$60 so should about twice as long.
So for twice the price, you should get a unit that lasts twice as long (15 years or more by my guess), has more overhead in the amount of power it can deliver, and also has a 80+ Gold efficiency rating. I would have to do the math, but the extra power efficiency of these units over the $60 ones has to save $70 or so on the power bill sometime in its 10 year or so life span to pay for itself.
