1) The form factor of the Dell XPS420's power supply is an ATX form factor.
(The term Form Factor actually refers to Motherboards, but has been used for computer parts such as the Power Supply, and the computer case)
The ATX form factor for an ATX power supply is approximately 6 inches Wide, 5-1/2 inches long, and 3-1/2 inches tall. (6 X 5.5 X 3.5 inches)
The stock 375 Watt PSU (Power Supply Unit) is held in the computer case by 4 Philips head screws, (back side), and two metal clips.
With the new PSU you can just use the 4 screws.
450 Watt minimum?
Know this; A computer ONLY uses the power it needs, and no more.
If you installed a 10,000 Watt unit, and the computer only needed 100 Watts, it would use ONLY 100 Watts. (Typical power consumed by a desktop computer when surfing the internet)
Do I assume that you're going to install a better graphics card?
If so why not avoid hassles in the future, and install an economical PSU that will be ready for future upgrades?
I would recommend an OCZ StealthXStream 600W.
1) If the temperature of the PSU is kept at, or lower than 118.6 degrees Fahrenheit ,(48.2 Celsius) the PSU is capable of putting out 736.2 Watts reliably,http://www.hardwaresecrets.com/article/OCZ-StealthXStream-600-W-Power-Supply-Review/451/9
(The above is the 9th page of the article. Here is the 1st page,http://www.hardwaresecrets.com/article/OCZ-StealthXStream-600-W-Power-Supply-Review/451
2) It has all the power cables you'll need. (Click on the Details tab)
3) It has a 120mm fan mounted on the bottom, and mesh on the back. Better airflow through the computer case.
4) The price. The one in the link is a Re-certified unit.
This means the power supply was sent back to the factory to be tested to their specs. Once found to be good, it is re-certified as a good unit.
(Could be someone bought the unit, and didn't want it. Newegg then sends it back to the factory to be checked out. They don't want to sell a bad power supply)
Dell Support, XPS 420 main page,http://support.dell.com/support/edocs/systems/xps420/en/OM/HTML/index.htm
You may find - Removing and Installing Parts - helpful.
BEFORE you begin:
Computer unplugged from power, observe Anti-Static Precautions.
Your body carries Static electricity. Static will fry out (Short Circuit) the hardware components inside a computer.
Relieve your body of Static before working on your computer.
Computer unplugged from power, and on a table, computer case open.
TOUCH the metal frame of the open computer case BEFORE reaching inside the computer.
IF you walk away then return, be SURE to touch the metal frame again.
For additional questions please post in a Comment.