I just wrote a full explanation since I have studied this topic and when I sent it, it went into oblivion becuase there were, according to the message I got, too many people on the site.
This is the short version:
Soft Sampler Libraries:
Vienna Symphonic Library - Very large and expensive. Must have Giga or EXS24 compatibility. Giga is only for PC, EXS24 comes with Logic and can be used on Mac ( not sure if Logic runs on PC but I don't think it does).
Giga -software sampler not sound library or synth. Giga's big claim to fame is direct from disc playability allowing use of samples that are too big for todays largest RAM allocations. Giga is only for PC.
EXS24 - Part of Logic and can be run on Mac. Many of the top libraries are made for Giga and EXS24 thereby covering PC and Mac users.
Halion - Delf-contained string library (as far as I know). Has gotten good reviews. May only be for Mac but might be for both.
Direct From Disc - As stated above, allows samples to play direct from disc for the largest and most realistic samples. Nevertheless, there still is a need for large RAM for performance functionality. Libraries like Vienna Symphonic Library by Ilio (hereinafter VSL) benefit from alot of RAM and will strain todays largest RAM allocations. This is allegedly going to be alleviated to some degree becuase of the advent of the Mac G5s which will double the limit on RAM.
Synths - Emu Proteus or Roland synths were the preferred for symphonic sounds. They are still used to some degree however, the Triton and Motif have taken away some of the thunder with updated samples. You might consider the Motif Rack or, something worth looking into would be the Roland rack mountable soft synth player. I do not recall the model number or name but this looks like it will prove to be interesting and might give you access to the best of the Roland sounds and there are many pros that still use the Roland string sounds.
Good Luck