I've seen and cleaned plenty of dusty computers while doing on site repairs, the best one so far had the entire base of the case filled with cat fur, but these pictures surpass them all.

I'm not sure how people can live with that, I tend to clean my case every 2 months and even with 6 fans inside, the dust is still minimal.

This is a general rule:
Intake - front, side and bottom
Exhaust - rear and top

The other question is already answered and yes, it really is just that easy to figure out which way it's blowing.

some of these are just dirty.

Ahh...Ahh...Sorry about that Chief...:-)

Awesome thanks much! Just checked and confirmed my front fan is intake (LOL thanks for the tissue paper trick - a quarter piece of graph paper I had on my desk did the job just fine - I feel I should have been able to figure that out on my own but I guess common sense was a bit lacking ;) ).

Some of those keyboards... I'd have to cut off my fingertips after using one of them!

Got some ideas for my next build from this thread! For instance, I never thought about using the tower as a monitor stand.... Cool!

They'd have to be certified for biohazard use.