If you want realistic orchestral sounds, the best bang-for-buck is East West Quantum Leap----the Gold package is a very good deal. The quality is truly "pro". I have used other software like Garritan Personal Orchestra which is really affordable and sounds quite good, but I've had ortchestral mock-ups I did with that rejected--when I re-did them with EWQL I was on the money. The articulation possibilites, plus the way it was recorded make EWQL one of the industry standards. I use Platinum now, but Gold is a great way to get started.
If you don't need that degree of realism and control, then try Garritan.
Or, if you just want an instrument that has some amazing string, pads, synths, etc. presets (and some editing capability), look at Atmosphere by Spectrasonics. Turn on the TV or watch a film and it's quite likely you'll hear the East West stuff and Atmosphere working. The Vienna libraries are incredible and used by all the top guys too, but VERY expensive.