I have ONE VisionTek 450watt power supply that mount in a 5.25 drive bay. No box.
This is a devoted video card power supply and is in addition to you routine power supply,.
and it's switched on when you turn your computer system on via a four-pin molex. It will have it's own power cable running out the back of the computer. It one just has a few hour on it as I never ever really used it, the PSU test great and stable but I have no means of screening wattage or load numbers.
If you have an off the rack computer and wish to run a luxury video card, you can simply include this to power it without burning or taxing out the stock power supply.
$30 Cash.
PS. When getting, I can link the tester to show it works and is stable.