Saw this as a printed article so I typed it up and figured that you all might find it usefull. If you have any different information, or updates or additions please post them below, I think that this could come in very handy to all involved!

----

How much power do you need? If you're running a micro-ATX motherboard with embedded graphics and audio, and a single 5400rpm drive, that wimpy 150W power supply might just do. But if your rig pushes half a gig of RAM, dual procs, Ultra2 SCSI RAID, and a GeForce 256, even a 300W supply may leave you bent.
We've compiled a list of roughtly how much power is consumed by common parts. By now you should know not to judge your power supply by overall wattage alone, so look at the demands on each separate line. Nonetheless, when looking at overall watage, add the requirements of every device in your rid, then multiple that number by 1.5 (because you should be taxing from 30% to 70% of your power supply's capacity). Some of your individual components may actually list their specific power requirements, so use those numbers for more accurate calculations. Always remember, if you need to figure out wattage, multiply the voltage by amps.

Code:
High-wattage AGP Card           20+W      +3.3V
Average PCI card                5W        +3.3V
Cached SCSI PCI controller      20-25W    +3.3V
Floppy Drive                    1W        +5V
10/100 NIC                      4W        +3.3V
50x ATAPI CD-ROM                10-15+W   +5V & +12V
50x ATAPI CD-ROM                10-15+W   +5V & +12V
8x4x32 SCSI CDRW                17W       +5V & +12V
SCSI CD-ROM                     12W       +5V & +12V
RAM                             10W/128MB +3.3V
Ultra2 SCSI PcI Card            5W        +3.3V
5400rpm IDE Hard Drive          5-11W     +5V & +12V
7200rpm IDE Hard Drive          7-15W     +5V & +12V
7200rpm SCSI Hard Drive         24W       +5V & +12V
10000rpm SCSI Hard Drive        10-30+W   +5V & +12V
Motherboard                     20W       +3.3V & +5V
550MHz Pentium III              30W       +3.3V
733MHz Pentium III              23.5W     +3.3V
300MHz Celeron "A"              18W       +3.3V
600MHz Athlon                   45W       +3.3V
Also don't forget that LCDs and GPSs and even the keyboard (0.2-0.3W 5V) and mouse (0.1-0.2W 5V) take power, USB adapters too.