Well, a 7200 rpm drive will certainly give you a little more speed - and if you are going to be editing large audio or video files directly on your machine (ie not from the source file on an external drive), i would say its a very good idea to get the speed upgrade, and $150 is a fair price. Bear in mind about the returns policy though....
...and that editing and saving large video projects to your system drive isn't exactly recommended, I'm sure most people don't have too many problems. Personally, I would always use an external firewire drive for source files.
Applecare will fix it if it goes wrong, but on a stock machine, you can return it before 14 days for any reason.
If you swap out the ram yourself, keep your original sticks just to be on the safe side.
Ned is right, the Mac Pro can be a much more powerful machine, with double (or even quadruple) the processor cores. If most of your time is spent editing big chunks of video, very large photos and graphics or very complicated audio, you will notice the difference. You can put up to 16GB or ram in, and the ram itself is quicker and error correcting, which makes a difference. They are more expandable, and you can put some really serious graphics cars in should you need to for 3D work and gaming, or if you want to use many different monitors.
On the down side, they take up a lot of room (too big in my opinion), and are a lot noiser than the MBP laptop.