I held my breath as I order a new PS for my XPS 8100 too. There's not much information about the size of the PS.
There are two issues I ran into. First, there are two clips, on the side where the internal cables come out, that hold the PS in place. Second, if your new PS goes past the two cllips, it then bumps into the cables for the USB/MEDIA slots in the top of the case.
I order a LSP650Ultra LSP650 650-Watt Power Supply. It was too long, however, with a pair of plyers & side cuts I was able to remove the clips. The cables are of course smashed a bit, but it is working OK.
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