In my opinion, GPO is a very good orchestral library considering its price. I've had it for a couple of months, and I haven't had a whole lot of time to play around with it because of school and work, but from what I've played with, I've been quite happy with it for writing orchestral pieces.

One thing with GPO is that it presupposes you have a MIDI keyboard given the way how the default play controls are set up. You use the keys to trigger the sounds and vary the attack, but you use the mod wheel on your controller to control volume and expression. Thus, getting a controller is all but a necessity when working with GPO.

As far as which controller, there's a multitude out there, all of varying in terms of quality, features, and price. I personally own an Edirol PCR-M50. It's 49 keys and has a good number of rotary pots, sliders, and buttons so that I can do much of my MIDI programming without having to resort to using a mouse. The action of the keys is decent, I suppose. I don't mind it and have gotten used to it, but there are others out there who can't stand it. If you can, I'd go down to a local music store that sells MIDI controllers and play on a few of them to get an idea of what you like in terms of feel. Other controllers you'll most likely see in this price range are Maudio, Alesis, Evolution, and Behringer.

After some internet surfing i got some names which sound interesting to me

Emu xboard 49
Esi keycontrol 49

anybody has experience with one of these 2 keyboards? what's good about them? what isn't?

on http://www.thomann.de/de/midi_masterkeyboards.html are some nice keyboards too.

i'm specificly looking for a "49" keyboard since "66" midi-keyboards become too expensive

anybody has more sugestions, please say!

I have an E-Mu XBoard 49. It feels good to me, nicely weighted and quite sturdy. You get one expression wheel suited for pitch bends and one more suited to volume changes and the like, plus 16 assignable knobs. It supports USB or MIDI so it's easy to get connected. I like it, although I am not much of a keyboardist so I can't really comment very authoritatively on its qualities as a playable keyboard. As a controller for writing and mixing music in Fruityloops however I am finding it invaluable.