I've been hearing pretty good stuff bout the Garritan Orchestra Library too, however i think u r looking for something like Nexus. Easy to use and has some really trancey patches. Indivision used to rinse them.
Trance synths are pretty simple, usually jsut a bunch of Saw's, with slow filters, lots of delay/reverb etc, better to figure out how to make the sounds you want than rely on presets, most synths are capable of producing most sounds with just a little bit of knowledge about how they work, look up Subtractive Synthesis Tutorial in YouTube to get a start
---------- Post added at 21:32 ---------- Previous post was at 21:31 ----------
if you really cant b arsed to learn synthesis, try Albino 3, plenty of trancey presets in there