Maxtor, Seagate, Western Digital and IBM are just a few companies that produce Hard Drives. The Hard Drive is a Small Permanent storage device that sits inside your computer and is the place all your programs are stored. Because of this, It's good to really get a dependable large storage device. Another reason for the size is because programs that are being introduced on the market are getting larger and larger. If you can get the system with 128MB of Memory it would be good to get it.
Memory speeds up the way the computer trades off programs when working in Windows.
Those trade offs include fonts, graphics, opening a few or more documents in your favorite work processor and of course how fast does it take to get going in windows. There are actually several different types of memory which can be confusing in itself. Older computers used a couple different types of Memory that in most cases won't be upgradeable into your new PC's. The memory is pricey however does well for applications such as Digital Video Editing and CAD to name a couple. DVD Rom devices will allow most all of your CD-Rom's to work in the drive as well as DVD Movies.