The +5V Rail soley supports your average PCI card, the Floppy Drive, and a Pentium 3 processor.
The +5V Rail also helps out with a SCSI Controller Card and the Motherboard, with help from the +3.3V Rail as well.
The +5V Rail also helps out the +12V Rail in supporting the CD-ROM, DVD-ROM, and Hard Drive be it IDE, Serial ATA, or SCSI.
Note: Remember to allow for roughly 8 watts per 128MB of RAM for your +3.3V Rail ( at peak ).
ie.) My stock Dell Power Supply had a +3.3V Rail that was puny, only able to support 46.2 watts.
As you can see from above, the RAM and AGP Slot are supported by the +3.3V Rail, and if you are using a "gaming" type card with more RAM, the +3.3V Rail can be easily overwhelmed.
My new PC Power& Cooling "Silencer-400" has a +3.3V Rail that supports 132 watts!