Windows XP - Pretty fast from hibernate (if you have new hardware)
Windows 2000 - A little slower than XP but I like it more.
Windows 98 - Get 98lite to make it faster booting.
Windows 95 - pretty fast
For all windows OS's - make sure you disable any network cards or give them ip addresses or it will hang for a while trying to find the DHCP server
Linux - Fast booting if you know how to strip it down and configure everything (check out the CAJUN project)
BEOS - Fast booting but limited hardware support and limited software support for controling it
MS-DOS - Fast booting limited hardware support (check out MPXPLAY for a good player for dos)
Also remember that a good part of your bootup time will be in BIOS so you will want a board that spends as little time as it can there.
If you get an auto shutdown controller you will not have to worry about shutting it down because it will push the power button when the voltage cuts out on the acc. line and then give it time to shutdown and then turn off the power to it.
as you can tell I hang out at
http://www.mp3car.com a lot also.