I drilled a 100mm diameter hole through to the outside of the house and have a 120mm pc fan blowing air directly out from the cupboard through that. Possibly not an option for everyone, but as a householder with power tools, it seemed like a good idea.

The PC itself is just a motherboard screwed to a flat shelf, with a bracket to hold the graphics card steady.

Works well most of the time, although in recent 30'c ambient temperature, it got up to around 37c in there when I was playing a modern game. My CPU is only 65w but I've got a new graphics card and that creates a lot more heat when it's working hard.