if you're adding another hard drive, you'll see them in windows as 2 separate drives like a C drive and an E drive, you'll still get warnings that either drive is full if it is indeed full. You can now use one drive for programs, and the other just for storage.
You could make the 2 drives into 1 effectively via RAID, but that's best to do if they're identical hard drives. So, don't worry about it.
Maxtor and Seagate are the big guys, and make quality stuff. You should alsk know whether or not you want to buy a SATA hard drive or an IDE hard drive. If you don't know the difference, look at the cable that your present hard drive is connected to
SATA is newer and a lot faster.
And you should also check on your motherboard to make sure it supports multiple hard drives. (some of the less expensive ones don't)
Look to spend about $100+ for 300GB+