Hi Stein - here are some answers to your questions:
-------------------
1) Do I need to have a hard drive (either internal or external) installed before I can add the styles to the Tyros 2?
-
Not really - you have internal space available (RAM) although it is limited. You can run all day accessing and saving styles to a USB thumb drive, but they come up quicker when you have them internally or on a hard drive. I installed a drive - it's not too hard to do.

------------------
I will more than likely use an external hard drive because it will be so straight forward....
-
Haven't tried that (I use USB thumbs).
------------------

2) but what benefits would the extra RAM give?
-
The extra RAM will allow you to store styles, registrations, and sampled sounds if you want to load any of your own custom samples or those from a commercial source. Without extra RAM or a hard drive, you will end up having to store most of your stuff on a thumb drive or other USB device. Not a show stopper, but it is much nicer having the stuff stored internally.
----------------

You won't go wrong if you get either a T2 or the PA800. I have a T2 and I am very happy with it. Lots of great sounds and excellent styles. Also the interface is very user friendly.

Hope that helps - good luck and happy holidays!