Power Supply mounting confusion

OK, I've gotten the bug to build my own computer. Having never done this
before I have been lurking on this group for a while and I've been
searching the internet for information. I have decided to build a computer
based on the Athlon XP processor. I have found that AMD, in their
"Builders Guide for Desktop/Tower Systems" case guidelines, recommends the
use of a "bottom air intake" power supply.

http://www.amd.com/us-en/assets/cont..._tech_docs/260
03.pdf

The logic of this seems obvious yet I have been unable to find a single
power supply with specifications stating that it brings in air from a
bottom intake or fan.

Many power supplies have a fan on the top. I'm unsure why since most of the
cases I've seen do not have a hole on the top to match. Then I thought I
would just turn the power supply upside-down, but no, the mounting screw
pattern is not symmetrical and the few cases that show the hole pattern
orient the powersupply with the fan up. I did find a site that shows how
to butcher your "incorrectly" built case to fit an upside-down power
supply.

http://www.directron.com/powermod.html

I would have no problem with altering a case or for that matter the power
supply to mount upside-down but seeing as they went to the trouble to make
the screw pattern different top to bottom I figured there must be a reason.

Am I missing something?

Do some cases have the mounting holes oriented so that the power supply
mounts upside down?

Could anyone here point me to a good 400+ watt power supply with a bottom
intake vent or fan?

Thanks

Charlie