Moisture was my first thought too, but - if I'm thinking correctly, it only condenses on something colder than whatever the dewpoint is. And the PC's will be running always warmer than ambient temp - meaning they will likely never be at the dewpoint itself, and not likely to build up condensation. But if they did, on a really humid, cold day, there is always that possibility. I think it might pretty much fry them if it happened.

It might be better than an attic for cooling, but how much better? Even if the garage is only a little hotter than it is outside in the shade, it might be enough for problems there as well. A CPU fan can't cool anything below ambient temp, and if it only has hot summer air to throw at the CPU it might be too much to keep them cool. I could imagine the case temperatures getting very hot. You might look at Peltier cooling, but you get into that condensation problem again.

I think some filters and such would be easy to set up to keep bugs out though.

What about an old refrigerator? A mini fridge, from a college dorm or a bar. I converted one to keep beer kegs in, perhaps you could modify one for PC's? Seems silly, but what-if? You'd have to set the temp a little higher than you'd want for food, but if the fridge could keep up with it the PC's would stay a nice constant temp. This wouldn't work inside the house, as the heat off the back of the fridge would be even more than the PC's create. But in a garage it could do it.

Also, on the case idea, maybe you could have a cabinet to put them in, inside the house yet, with a dust filter and air intake hole somewhere. Place the PC's on something soft inside to catch the noise a little, and with the cabinet door shut - you might hear them - but it would be a lot quieter than now. Only problem is heat - but maybe a duct and a fan could blow the heat outside, something like a clothes dryer does now, but with a better screen to keep the critters out.

That's all the silliness I can come up with!