How can you tell if a psu will fit in a dell?

[Druid]

My friend has like a Dell inspiron 530 that he customized up, and he wants to upgrade his vid card, so he needs a new psu.

I was gonna tell him to get this: Newegg.com - Antec BP550 Plus 550W Continuous Power ATX12V V2.2 SLI Ready CrossFire Ready Modular Active PFC Power Supply - Power Supplies

but I don't know if it will fit.

 

[Muffin Man]

ive seen this one before, which is supposed to be a psu specifically designed for dells, even though ive heard that they are the same internally as the regualr 500w, just slightly shorter. this could be an option if you want to stay safe i guess, even though it looks like any regular sized atx psu should fit in the 530 case.
PC POWER and COOLING PPCS500D 500W Active Power Supply Retail at ZipZoomfly

some info if you want to look.
http://www.pcpower.com/products/description/Silencer_500_Dell/index.html