Alrighty Pixie, I'll try.
The Arcade package comes with, well, arcade games. And that memory card, which is fine if you don't plan on getting Xbox Live. The only reason I say this is because if you do plan on getting Xbox Live, you'll be downloading demos and TV shows and what not, which takes up a lot of space. My 360 didn't come with a hard drive (the Core package), so I got a memory card and did fine. Then I wanted Xbox Live, so a year after I had bought my 360 I got a hard drive and Live.
It really comes down to weather or not you want Xbox Live, which is really the only reason I see you needing a hard drive. If you don't think you're getting Live anytime soon, grab the Arcade package and a sweet game and play that. Late on you can pick up a hard drive used, like I did, for $70.