One important thing that people forget is to clean your casing regularly. Turn off the system and clean out the ducts.
If you really want to use a vacuum, dismantle the housing and clean that. Don't vacuum the computer components.
To clean the interior and get rid of dust, use a can of compressed air (I get mine from Challenger, probably the only thing I used to buy from them)
Just make sure you clean the system, to ensure air flow.
Really depends how you store your box. But rule of thumb, once every three months should be fine.
Cards tend to fry due to poor air circulation and the heat builds up really fast, especially with a clogged system. (not like the old days, gfx cards didn't have the load we put through our cards nowadays with gfx intensive games and applications)