Collegestudent,
I think your on the right track but your slightly misguided. The "RAID" that your thinking of is more commonly known as "Dynamic Disc" in Microsoft Speak. Dynamic disk allows you to plug in drives into a spanned volume of disks and have the OS automatically resize your drives in a non destructive manner (aka you dont lose yoru shite).
Unfortunately since you already have stuff on your main volume you cannot make a raid 0 volume without having to repartition your volume as one (lose your stuff).
to answer "spindleX"'s comment, you do not necessairily need 2 drives of the same capacity, although, if you have a 40 + 60GB raid0 volume your total storage will only be 80GB because Raid0 will can only double the smallest drives size.
Wikipedia: RAID
Wiki will help you understand exactly what RAID is.
now, to solve your problem i'd reccomend setting your new drive up as an independent and separate volume and dump your docs, music, movies etc.. off your boot drive and onto your storage drive.
You mentioned that when you try and "move" files it only copies them... well, the obvious solution would be to copy them to your new "dump" drive and then delete the (now) redundant copies from your boot volume.
I would strongly reccomend against you using RAID (even IF your willing to format both drives.) RAID is not for the average joe, using raid0 you double your chances of a catastrophic failure (aka OH %$#& I LOST ALL MY $%&(ing STUFF!) because since your stuff is spanned over 2 drives, if one goes, your SOL.
Read over the Wiki link very carefully and understand what your getting into. I've had one too many novice customers try and create and use raid0 volumes and botch thier partitions.