Using Raid is an excellent way to speed up your harddisk access speeds if you have more then one harddisk and they aren't running at full bandwidth.  RAID is where you split up the files so that odd bytes are on one, even bytes are on the other.  The advantage is that since have more harddisk heads working at the same time, you increase the speed of your harddisk.  IF you are using two harddisks now in windows, what happens is that only one harddisk is working at a time.  Why not use that 2nd drive to your advantage and set it up in a RAID array?  There are two types of RAID controllers.  Software and Hardware.  Hardware are more efficient but more expensive.  Software are cheap but are less efficient and wont work in windows.  However, RAID controller software is included free with Linux. THe price for a Hardware RAid controller is normally $2000 or more, but a hacker recently discovered that a IDE controller on the market that sells for $70 is capable of doing Hardware raid.  If you are willing to use two harddisks to your advantage and set them up in Linux, read on.  Otherwise, buy a RAID controller... If you are crazy enough considering Linux is free. 

To use Raid with linux, you must have more then one partition because the boot information cant be loaded off a raid drive.  You must partition either a Linux Native (EXT2) drive or a REISERfs drive (only needs to be about 200megs) and make the mounting point /boot .  DONT USE A RAID DRIVE FOR /BOOT OR LINUX WONT WORK!  Next, partition a raid drive on both of your harddisks using the software provided.  They should be both the same name eg "MD0".  Usually another tab opens with the new raid drives name and you must set the mounting point in that to "/".  remember to make a swap drive.  Also, MAndrake will only warn you once to put /Boot on a non-software raid drive.  so be warned.  Check the space of the raid drive to see if it is Drive  A's Partition+Drive B's Partition.  If it isn't, just remake the drive. One final note is that I recommend that these two harddisks are on seperate IDE's because that will speed it up much more. Then, just sit back and enjoy the fact that you are making your computer more efficient. If you wanna have raid in windows, I recommend getting that $70 ide controller, BUT you better do research to confirm you are getting the right one!  Alot of motherboards also have built in RAID controllers as well.. Maybe u already got one ;)