Keep in mind the Mac Pro uses some very high end memory, known as
FB-DIMM. Third party stuff might be slightly cheeper then Apple charges, but make sure you get ones with proper heatsinks. FB-DIMM's generate a lot of heat.
Hard drives, any SATA one will work. The Mac Pro comes with 4 bays, and 3 will ship with empty brackets. Simply buy a SATA hard drive and put it in the bracket, and you should be set.
And yes, buy the AppleCare plan to extend the warranty to 3 years, if you think you will own it more then one year. While the general reliability is good, if something like the mainboard dies, it's not going to be a cheep expense to fix. You can wait and buy it later though, as it is valid as long as it is activated during the first year of ownership. Also, it is heavily discounted for students, so if you have a college friend, they could help save you a bunch of money.