It really depends on what kind of mood you are trying to set and also what kind of game you are working on. There are certain situations when an orchestral score might not completely cut it for a certain mood or setting, that's where synthesizers come in. There are a few softsynths I use that come close to an acoustic sound, but there is always a little telltale nuance in the sound that automatically tips you off that it's coming from a synth. There are a variety of sampling software and sample libraries out there which contain samples of actual orchestral instruments in a variety of different articulations, even these have setbacks though. They can still sound fake even though they are sampled from the actual instrument. The trick for realism lies in the sequencing itself. If you sequence an orchestral piece in the exact same way you sequence, lets say, a techno piece, it isn't going to sound like a real orchestra (playing wise). The real power, as was stated, is in the hands of the composer.