Well it depends on which RAID Setup you want.
BTW: For anyone who wants to know. RAID stands for (Redundant Array of Inexpensive Disks)
Anywho...back to the question
If you want a nice speed and performance boost then RAID 0 is the way you go. RAID 0 is the Stripe Array whereas RAID 1 is mirrored (if you know this already sorry, but its also for anyone who reads this in the future)
RAID 0 will take two hard drives and basically combine them into one so therefore it'll be like you have a 14,400 RPM HD in a sense. This array will write data segments to both of your hard drives so it can access the same file from two different hard drives which means the file is brought together quicker, because you have one half being located at the full speed of HD1 and the other half at the full speed of HD2 so it makes for much quicker, loading, copying, and stuff like that.
RAID 1 is a Mirrored Array which is completely opposite of performance boost, but is basically a fail safe choice. It writes the data twice, once to HD1 and once to HD2, this case in the event that one hard drive fails, you won't lose any of your information and the second hard drive will immediately take over...
Now, the tricky part of this is, is that a RAID Setup is somewhat a pain in the ass....
You can't simply switch your current setup to a RAID array however. You have to actually format your drives and start clean and set it up from there.....also I believe your two drives will have to be identicle
I don't know enough about RAID though, and I'm going to have to ask one of the other mods, because there was a sticky on here fully explaining how to setup a RAID array and it's a lot more difficult and lot more bs involved than you'd think....
I do know the answer to the 'can I just unplug it?' ....You should be able to do it under a RAID 1 setup, but under RAID 0 since half the information is on one drive and half the info on the other, if you deleted your RAID setup you'd have to do a complete format and install to return it back to normal...under Mirrored setup though, then yes you should simply be able to go into the RAID Setup menu and disbable the Raid controller so that it no longer copies files to drive 2 and you can use your main HD as it was
EDIT: Bah! Twice today like 4 posts have gone in by the time I get my one up lol
EDIT 2x:
I can't use RAID on a single drive?
No you can't as the whole purpose behind RAID is for the specific purpose of either combining two drives into one or creating a fail safe kind of environment.....it seems you want a performance increase so you'd want RAID 0 for striped.....otherwise theres no need for a RAID Array and one HD completely goes against what the RAID stands for lol (look above for my nice description of the actual meaning of RAID
ehh here it is again : Redundant Array of Inexpensive Disks
keyword is disk
s 
