Dell Dimension 8400 PSU Upgrade

Hi newcomer and welcome to the Tom's hardware forum.

That¿s a good PSU, BUT bear in midn that the most of the DELL's PSU aren't standar ATX. So, give me the model (Dell inspiton 1520 for example) of the PC and we can see if the PSU that you want that fit on the case.
 

No, that's a replacement psu.
The stock psu in an 8400 was a 350 watt/28 amp Newton/Delta, and definitely real.