i think OEM is pretty much the only way to go. if you want retail hard drives or operating systems, you're lookin at a much much higher price tag. you'd have to go to one of those big stores.. (not sure what you guys have in the states... compusa?) OEM products are pretty much excatly the same, they just don't come in fancy packaging or w/e.. and as the other guy mentioned.. you might not get all the cabels and stuff, so check to make sure you're getting everything that you need.
as for operating system, OEM means that you can't get support from microsoft (i think..).. you'd have to buy the retail to get that.
"Is virtue a thing remote? I wish to be virtuous, and lo! Virtue is at hand." -Confucius