2.1 The iPod
For iPod, iPod (Scroll Wheel), and iPod (Touch Wheel), hold the 'Menu' and 'Play/Pause' buttons down together until the Apple logo shows up. For iPod mini and iPod (Click Wheel) hold 'Menu' and 'Select.' This won't erase any songs on your iPod so it's ok to do.
For more information, see the Apple support document.
OSes supported: Windows 98 SE (with firewire update), Windows 2000, Windows XP (all), and Linux (various distros)
Firewire Card: Most firewire cards will do the trick. Certain chipsets are preferred by advanced users. Personal experience has shown me problems with the firewire port of audigy sounds cards (by Creative). Installation of such devices is easy and usually only requires an empty PCI port (refer to your motherboard documentation). (Ed. note: Apple now has a list of Firewire cards that it recommends.)
Software: Interface software (filling your ipod with music), MP3 production:
I recommend EphPod. It has a nice interface it works relatively well and its bugs are remedied by frequent updates. Best of all it is freeware. Other options are Xplay and MediaCenter9 and Windows iTunes. The former two are pay options with trial downloads available.
I use CDex. It uses the LAME (3.92/3.93) engine to produce mp3s of relatively good quality. Superior to MMJB. Purests will argue but that is not for this discussion. The fact remains it will provide tagged mp3s that are free of pops, clicks and hissing.
M4A, M4P, AAC.
There is a lot of noise being made by Apple's adoption of AAC. AAC is mentioned here only as far as to confirm it functions on Windows iPods with firmware 1.3 (2nd generation) or 2.1 (3rd generation).
Here is a list of Firewire PC/PCI cards that Apple has tested and found to work with the iPod.
iPod: The Battery Status Indicator Is Approximate
iPodlounge has a very good one.
This FAQ should answer your questions.
Additionally, Apple has a new section on their website devoted to lithium-ion batteries, and specifically iPod batteries.
This site will explain how to get into diagnostic mode and the various tests you can run. There's a picture of what it looks like right here
A tip o' the hat to tmpchaos