They have air conditioning though to keep the air dry too, it is a humidity that concerns me more than temperature.

That's true, and a good thing. Having lived with a few computers on my boat for 5 years/winters and having learned a few things about humidity, moisture, midew and all that, I feel comfortable predicting that it won't be a problem unless the garage is seriously dank. As a previous contributor pointed out, all those water molecules will want to come to rest on the surfaces in your garage that are cooler than ambient temperatures. Your computers don't fit the profile unless it got very cold relative to the computer's ability to generate some heat.

If it was still a concern, I'd think about a dehumidifier, or just a simple heater with a powered vent to the outside -- humidity only goes away when you move it outside somehow. Here's a thought: Build a plywood manifold that fits the back of your server and attach a 400-800 CFM squirrel-cage fan to it with a short 3-4" duct to the outside. Your Dell becomes a dehumidifier!

Remember: install porn!