It's standard ATX, though Dell's design is taller than standard.

 A retail psu will work, but leave a small gap when installed (hasn't been an issue for anyone).

How does Dell choose if 375 or 425 watt power supply is in your XPS 420?

It's based on the video card you get with your system.

My system is running fine on whatever power supply they installed, which is all I care about.

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@LUUEEIIE wrote:
My system is running fine on whatever power supply they installed, which is all I care about.

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Based on you specs you have the 375 watt PSU.  I have the same Video Card.  Very nice card.

There was a post by Chris_M a while back that stated the 425W supply only came with the High perfomance system

My system came with a 420 watt power supply. Below are my specs,

 

Dell XPS 420
Window Vista Ultimate 64Bit
Intel Core 2 Quad Core Q6600
4 GB DDR 2 SDRAM 800MHz
2X500MB SATA II 3.0Gb/s 16MB Cache each in Raid0 (7,200RPM) 173 mbs read speed
1X500MB SATA II 3.0Gb/s 16MB Cache Single Drive   (7,200RPM)   54 mbs read speed

512 Nvidia GeForce 8800GT
Sigmatel Onboard Audio
16X DVD +/- RW DL

16X DVD +/- BD DL
19 in 1 MR with Bluetooth

4GB Sandisk Flash Drive (Superboost)

OS fresh install with reformat 

Intel chipset drivers  (latest version)

Nvidia x64bit video drivers (latest version)

I just chatted with the online Dell Sales Support and they said it only comes with the 420W when you order the 8800GTX.  Seems they have no clue then.

 

I'm looking at it with the Q9450 and the HD3870 so as to reduce power use and increase performance but can't get a clear picture of what PS I might get :(  I'd liek to know for sure as I'm trying to decide between the 420 or the 630.

 

Don't beleive there is a high perfpormance system in Canada thus the confusion on this.  I asked once before and I believe CHris wasn't sure.

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@schooner2000 wrote:

I just chatted with the online Dell Sales Support and they said it only comes with the 420W when you order the 8800GTX.  Seems they have no clue then.

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You are quite correct. Dell support has no clue. When I ordered my system, you would get the 420 PS when the 8800gt video card was part of the system, as well as 2 CD\DVD drives.

 

I also had a phone modem installed for special reasons I require. The system came with only one 500GB hard drive and I added the 2X500Gb in a Raid0 configuration.

 

I believe now, that the 420 watt PS is not available unless you get the 8800GTX card.

 

What a mess. Dell seems to be always changing their policies and offers, that leaves their customers with questions and doubt.

 

Hope this helps.