Ok. I really wish there was a sticky that outlined the compatibility issues of Dell PC. How many questions do we all answer on a weekly basis about this?
The PSU that DJ-CHRIS recommended although very good will not fit in that 4550 case. SO DON'T BUY THAT ONE. But PCP&C does make a PSU that will fit in a 4550 case and it's this one here.
The Turbo-Cool 425 for Dell is different then any other PSU in only one way. It was designed "without" a switch. Dell cases are designed with out an opening for a switch because their stock PSU'S don't use them. So in order to upgrade you need a PSU that doesn't have a switch.
There are other obvious issues that you need to know before upgrading a DELL PSU also. Like 20 pin or 24pin & 16pin vs. P4 pin. But, PCP&C has gone ahead and put together Dell Lists so you can match your model with a Dell PC for compatibility.
The Turbo-Cool 425w is 20pin & P4. You'll get 425watts continuous and peak power of up to 500watts. You'll get a solid 20a on the 12v Rail. This PSU has a lot of balls. Although it has a price tag of $149.00 it certainly is not only the best you can buy, but it's the best you can buy for a dell. And if you ever upgrade your PC and rebuild in the future, you can certainly use that PSU as I am also. You just won't be able to use it in a PCI express/SLI system. As a 24pin is required.
I've got tons of components in my new rebuilt system including a p4 3.4ghz, X800XTPE and (4) SATA HDD'S... plus a bunch of other things. And this PSU is solid. And still has enough balls to overclock.
If you can spend the cash... buy the PSU. You won't regret it.
Although, Buying a new case & motherboard and scraping the Dell 4550 mojo is probably the better of the two, don't forget, unless you have a full version if windows XP, your Dell OEM windows won't work with a rebuild. And oh yeah, you'll need to format your HDD and reinstall also.