The first step would probably be an investment in Native Instruments' Kontakt. It's a much more reliable and stable platform for sample playback, and the most widely-supported sampler format. There are lots of great-sounding symphonic libraries for it, such as those from
Spitfire Audio, VSL and
Project SAM.
But because it's so widely supported there are also plenty of other interesting libraries, including pianos (straight, bowed or plucked, grands and uprights), percussion and rare or unusual instruments. Best of all, many of them are very inexpensive.
Note that some libraries can be used with the free Kontakt player, while many others require the full Kontakt product. It's the latter group that are less-expensive, as they're not passing on a licensing fee to NI, and the reason I recommend purchasing the full Kontakt.