i would suggest making some sort of trolley and then some way of securing the case to the trolley. permanently attached wheels make it a pain to work with the computer.
i saw some mini skate boards at a toystore once, they may work fine.
make sure whatever you get can support your computer. say, 100 - 200 lbs total, or 4 casters that can support 25 - 50 lb each.
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i took a look at the wheels from directron, i do not recommend using those. i recommend a single piece wheel'd type caster if you are going that route. they even make casters out of rollerblade wheels, so it's a transparent "tread" and looks really nice.