Yea? All midi?
I thought that Dimmu usually hired some ensembles to record the stuff. Of course combined with midi/samples/synth etc. That's the way i would do it. (if the budget is there ;-)
I usually do the "poor people" workaround:
I track/produce all that stuff in midi using the Bands Keybord (most of the time a Korg Triton) - later i record everything into the DAW - multilayer sounds usually split to individual tracks (for instance strings deep, strings melo, choir1 male and choir female and so on).
Then i fire up the Gigastudio System and go through the stuff, experimenting with sounds, i often programm some extra strings and stuff like that in Giga.
I can highly recommend Vienna Symphonic Librarys. They are shurely one of the biggest and most professional (but very expensive) Librarys availeble these days.
I have Gigastudio Orchestra (the big pacage) - a "Lite" (but still big) version of the Vienna Stuff is included, that usually is enough for my needs.
The most important thing when programming strings is the arrangement (not everything plaing the same notes etc) - you should try to get a grip on classical orchestral stuff, the recording etc. I record classical music here from time to time, i like it!
You propably want to layer loads of sounds bacause that is how an ensemble/orchestra works. Be carefull with panning!! Take a look at a picture of a real orchestra and try to understand how they are placed and why they are placed that way.
Using the proper reverbiation is important was well.
HTH
brandy