Do you know what motherboard you have in your system? Usually its written on it, or there are programs we can ask you to download which would give you this information also.
I want to know by the motherboard model, what your motherboard supports as a drive, i.e, if it supports IDE, and SATA/SATA II, or just IDE, etc, so we can choose the appropiate and best drive for your needs.
Also, how big of a drive would you want? They are pretty cheap now, for even one as big as 320GB-500GB depending on what you'll use it for.
I used to have just a 40g drive and was in a similar boat to you so I got a 160G Samsumg IDE drive then set that up as my system drive and keep the 40G (C)as storage, it also has my system back up of (D) on.
Like everyone else said, there are a ton of options. If you don't wanna bother with installing an internal and you're mainly looking for storage, an external hard drive will work perfectly.
I surpose you could just get like a 8GB USB key stick for quick storage but if you not wanting to run and OS of it at a later date then as external drive would be sufficient as other people have mentioned.