Here is my response:
PG music has some pretty decent stuff built in to Real Band, like the free Sample Tank synth that comes with it (IK Multimedia) and ye olde Coyote for very basic stuff. Moving beyond that you may want to check out Garritan's sample libraries for piano, orchestra, and Jazz Band. The Garritan Personal Orchestra and Jazz Band samples are great. The Abbey Road Grand is amazing. All compatible with Real Band. Garritan libraries can also be bought in packages on the PG Music site. Google Garritan.
If you play the notes yourself, any decent keyboard with a midi out will do, because you are only looking for the keys to drive the software synths, such as the Aria Player. Obviously in this case, you are playing the keys into a midi track in Real Band or another DAW and assigning an instrument patch from Sample Tank, Garritan or another software synth. (Soft synth.)
If you are simply taking BIAB generated midi tracks such as strings, you probably still want to run them through a player such as Aria if you want a real piano sound, for example.
I use Sonar to produce in, and that comes with a lot of free and excellent built in synths that are probably more than you will ever need, if you also have some classical stuff like Aria. The Session Strings in Sonar are actually very, very nice.
The learning curve on all these soft synths can be pretty steep but it is worth it once you figure it out. To start with, why not fool around with Sample Tank in Real Band and get a feel for how it goes? Then check out some of the other synths other folks have recommended, once you know what it is you want the most, orchestral strings, or Motown. Different synths suit different purposes.
As a side note, I am finding that is is actually easier to record and edit audio in Real Band than in some other DAWS. I think Real Band gets overlooked or underrated sometimes when actually it is totally awesome for recording a nice clean audio signal, and editing is a snap. I almost never get clipping with Real Band even if I have my amp turned up to 11 and the mic shoved right in front of it. Then once you have really clean audio tracks and Real Tracks you can move them to a DAW like Sonar that has all the fancy production stuff and more soft synths for midi than you will need, probably. The midi export dashboard in Real Band is a dream. Another piece of really sweet programming that doesn't get the praise it deserves.
I hope this makes sense. Tell me if it doesn't and I will try and be more specific. It takes a while to learn the ropes with midi and soft synth production. It isn't always easy so if you're frustrated any at all, you're in good company.
