There isn't a "one answer solves all" solution to achieving the best performance with VDL:2. All computers have different specs, so the more you learn to understand how these specs affect your end result, the better equipped you'll be to make good decisions.
First off, RAM is a very important component to using VDL:2, particularly if you plan to load many instruments at the same time. The system requirements state that the bare minimum RAM for VDL:2 is 512mb. Keep in mind, if you only have 512mb RAM in your computer, it's also serving to run the operating system, Sibelius or Finale, as well as VDL:2. So 1GB is a good recommended starting point if you want to load more than just a few instruments. 2GB is even better!
If you have limited RAM, be sure to download the DFD (direct from disk) extension from our Updates page. DFD allows you to stream samples from the hard drive rather than loading them purely to RAM. Using DFD will decrease the time it takes to load instruments, since the entire instrument isn't getting loaded completely to RAM. If you have a lot of RAM in your system, you may choose to bypass using DFD altogether which should perform very well.
A fast CPU (aka: processor) is also one of the most crucial components to optimal playback. There is a direct correlation between how many voices you can effectively play back and the speed of your processor. If you're in the market to purchase a new system soon, try not to skimp on the processor.
Good audio drivers will also improve your performance. Many Windows machines ship with cheap consumer soundcards that aren't necessarily geared toward churning out the audio you're going to ask of it. We highly recommend that Windows users use soundcards that ship with ASIO drivers as they by far perform the best. Macintosh systems all ship with CoreAudio drivers, which by default perform very well.
Another topic that has a big affect on performance (related to audio drivers and soundcard) is the latency settings you'll find in the Soundcard Setup window of VDL:2. Essentially, latency is the time it takes from when a note is requested, to the time it gets pumped out of your speakers. This is usually measured in milliseconds. Lower latency settings place a large burden on your CPU's ability to process all that sound so quickly, so generally, when using VDL:2 with notation software, setting the latency a little higher is a good practice to avoid possible pops and clicks. Setting it too high could distort rhythms, so you may have to fiddle with this setting a bit. 40ms is a good starting point.