For most VSTi and virtual synth situations, there are three things (all specs basically) that play into that:
-Processor speed/config (i.e. multi-core, speed, caching, etc.)
-System bus speed (faster the better for multi-threaded processes, such as multi-track processing/playback, synthesis, plug-in processing, etc.), and
-System RAM (used to load instruments into RAM for high polyphony/complex layers.
Apart from that, fast external hard drives for storing your recorded audio files (digital audio tracks) that are separate from the drives that host your VSTs/virtual synths/plugs…
Another aspect is the connectivity to external hardware. Unless you plan on housing all the virtual synths internally (on a dedicated internal SATA hard drive, connected to the main internal SATA bus), then to achieve the fastest outside throughput, you should consider having a system that has available cards/ports with eSATA or at least FireWire 800. The reason is this: what happens in many cases is, the manufacturer either sends you a DVD library that you load onto your own SATA drive (a dedicated drive for sample playback), or you buy a pre-loaded hard drive (as with GigaSYNTH for example). Either way, each drive represents a library of instruments or a single instrument.
Thing is, by the time you spec out a system that meets all these requirements, you’re well on your way to getting a Mac Pro anyways. I’ve seen some setups (both Mac and PC), where folks will actually do two systems/towers: a tower and dedicated motherboard just for samples/instruments/libraries, connected to a “host” computer that runs the audio tracks and DAW software. The two computers are tethered to each other via fibre optic cable/cards, and/or other high-speed connectivity (eSATA, etc.). Mac and Apple have a nice solutions for this, with the Logic Studio system, you can actually load your libraries, plugs, synths onto a second Mac, then connect the two computers together either by fibre, or even over ethernet.
Or if you see how the pros do it, A host system, with bays of PCI Express cards that are essentially “computers on a card” that do the vast majority of the brainwork, where the host computer just goes along for the ride managing system calls and RAM. If you look at the architecture of a Digidesign ProTools HD system, you’ll see what I mean.