I messured it and checked the specs on the western digital website. I already have it out (it is from a friends dell) so I have the serial number and what type of hard drive it is. It is a western digital Caviar WD600 60gig HD. The specs though aren't very helpfull, apparently their drives don't have exact widths? It say 4" +-.01". I figure a 3.5" drive should be what it goes in, seeing the dell I took it out of was a dimension 4600C, one of their small/slim case designs.
i re-pulled out the ruler, the drive is one with a -.01". it is around 3.9~3.8" in width
Great, so I've been measuring the drive plus the drives "enclosure" for lack of a technial term, something you aren't supposed to do. So I guess it is perfectly safe for me to order a 3.5" enclosure then. I have just been putting it off becaues new egg has a horrible return policy, so i don't want to order something I can't use.