I've found that a great reverb is essential to getting a great orchestra sounds.
I was using Logic's Space Designer with the PCM91 convolution of "Deep Blue" (pretty decent, but a little stiff. The real PCM is much more alive and 3D sounding).
I then started using the Powercore VSS3 verb "Large Clear Hall"
When I can, I use a Lexicon 300 "Large Hall".
Choirs seem to work well with convolution verbs (mainly churches)
I just picked up a TC Reverb 4000 based off of some pretty great reviews. I think it's gonna be great. The "Europe Orch Hall" and "Show Hall" both are pretty mind blowing right off the bat.
What do you use for your orchestra?