If you have to use shelves I recommend the heavy-duty blackish-greyish Closetmaid shelves. I have some of these in my garage and store extra equipment on them. Right now I've got an SE, a monitor, a big old Compaq, and a few printers on those shelves with no signs of sagging.

They're easy to put up as long as you have the right drill bit. These are good for stuff you aren't preserving for aesthetic appearance but just need to get off the ground.

Of course, if I'm doing something in the garage like washing a car or doing a lot of sanding at my workbench, I cover the computer stuff with an old blanket that used to be my dog's. This also keeps them relatively warm so I like it when I'm going to be keeping the garage door open for a while in the winter, or if the lows are so low that they need extra warmth.