There's a couple of irrelevant responses in this thread I see- of course the ever-so predictable "nothing just live instruments" well that's not helping the questioner given what he asked so such answers are irrelevant, if not obvious! Of course live instruments will always be the most realistic, but it's very impractical to assume that anyone has a personal orchestra to their disposal.
Joshuac... gives a solid answer.
1. A notation program for inserting the notes. Finale and Sibelius are both productive, user-friendly programs. However, using that specific form of notation is not necessary. Many producers just enter or record midi data into a piano roll. Considering that you are producing music and directly exporting to a waveform, doing your final editing in a piano roll is recommended, as a sheet music leaves too much room for interpretation.
2. A Digital Audio Workstation- This houses your synthesizers, samples, midi & digital data you use for composing/sequencing/arranging, and the mixing board. Having one of these is not necessary, as you can do all that within a notation program, but to a lesser extent. You can replace your GM midi instruments with Garritan Personal Orchestra. You can also route the midi output in the notation programs to an external synthesizer or sample library but the dynamics, expression, and latency usually doesn't work as well as Garritan.
Reaper is a solid and useful DAW; I prefer FL Studio for it's high quality GUI and user friendliness. It's also one of the cheaper DAWs while still being very powerful. However, it contains poor stock samples and virtual instruments for making orchestral and realistic sounding music.
3. An Audio editor- eg. Audacity and Adobe Audition. This is mainly for in-depth sample editing and mixing and recording instruments. Not necessary; most DAWs have these capabilities anyhow.
4. A Sample Library- these are the actual instruments that generate your sound. They contain layered, real-recorded ("one-hit") samples of live instruments that while are no replacement for the real thing sound very authentic and are commonly used in film, television, and video game scores. They are not synthesized (although both methods may be paired together). These are great sample libraries, which are compatible with any DAW that offer third-party plugin support
Vienna Symphonic
East West and Native Instruments
Tascam Gigastudio
Edirol Orchestra
Miroslav Philharmonik
Garritan Personal Orchestra (as mentioned above)
if you want to go the free route, look up specific instruments in soundfont or .VST format. The first answer recommended DSK instruments, I second this- they have some decent quality ethnic/world related instruments that you should check out.
As a matter of fact, the above answer could be related to any genre of music, not just classical.