You just need another controller card for two reasons 1) usually the motherboard will have a single controller with two IDE channels on it. So you need another physical connection. 2) You don't want to share the channel with other devices for performance reasons. Serial ATA is a nice choice but I have had problems getting the MoBo's controller to work with it. So IDE ATA 133 might be simpler. Often HD kits come with another controller. All that being said, the only time I have seen more than 50-80Gb actually being needed (RAID aside) is for video editing. So you *might* want to ask "do I really need more" or can I clean and archive stuff. But hey you might have a LOT of data you create, which is entirey possible. I have a video machine with 1/2 terabyte on it. (200Gb x 2 and a 120Gb). All the rest are running 36GB main drives and a secondary slower drive for backup at 36 or 80 GB. Cheers!
There are two IDE connections at every mobo, i.e. you always have two masters and two slaves. If you have two HDs and one CD-DVD drive, then you can connect one more HD as slave. Otherwise you need extra controller card. I have two HDs (2 masters), cd-writer and DVD-player (2 slaves), and extra serial ATA HD (a separate controller).
I have had very good luck with the Promise controller cards. http://www.promise.com/product/product_list_eng.asp?familyId=3 But it is quite easy to convert everything from your existing hard drive without reinstalling anything if you want to give it a try. First add your new drive as a slave in place of your current second drive. Format it with the copy system files option so that you will be able to boot from it later. Next go to run and type in xcopy32/h/e/c/k/r/y C:\ D:\ (only leave one space before the C and one space before the D) then hit enter. This should completely copy everything from your existing C as the source drive to your new D as the target drive, operating system, programs, data, files, etc. Your desktop, email, everything should look and work just like it did with your old drive. If this will not work for you from the run box type it from a dos prompt. Next attach your new drive as the only drive in the system and see if it works. This way you run no risk of messing up your current hard drive. If it doesn't work (but it should with no problem) then you can just reattach your original hard drive and be where you are now.