The size needle you use (circular cord length as well as diameter) depends on what you're trying to make and what gauge you need.
I started out buying individual needles from Wal-mart for every new project requiring a new needle size...but after about three purchases, and a few days of looking online for patterns I wanted to make, I realized that every project I looked at required a different needle.
A little quick math told me that I either needed to seriously limit what I was knitting to only on those things that could be knitted on the 2-3 needles I had at the time, or I needed to bite the bullet and invest in an interchangeable set.
I went for the KP Options (nickel), and after a year or so, also bought the harmony wood set. For what you get, they are reasonably priced. I haven't priced the other sets in a while, but you might want to do some research and see if an interchangeable set will be best....or research the afghans you want to knit, and see what size the most common patterns call for, if you only want to get one or two needles.