I recently purchased a Shuttle XPC system and upon inspecting found that the included chip and speed was actually an Intel P4 3.2GHz -- but upgradeable.
The Dell XPS Gen 4 that my colleague Bruce was using and about 3-years old I noticed was similarly using a P4 3.2GHz.
The bunch of hardware I got from the office which was to be sent for disposal due to age to my surprise were also P4's but using the older 3.0GHz speed.
The larger casing I'd already tested and used previously as the FreeNAS case, works fine. The uATX/micro-ATX casings however both require a new PSU (and I'm almost sure thats why they got sent to the trash bin). These systems are more than 4-years old and thus were eligible for disposal.
Looking for a P4 TFX 12v power supply is a chore I discovered. Not many supply them anymore and I'm still mulling the decision to get a couple of these to complete those systems. Eventually they'll probably go up on Craigslist or maybe eBay.
Here are some sources I found for the PSU:
(1) Enhance ENP-2224B_2G - the original PSU used
(2) Acbel TFX/260W and 300W
(3) Memory4Less - also has the Acbel PSU
(4) Sparkle Power - has the Flex ATX and TFX options