I sent this to sndfreQ. This is right up his alley. It might be awhile so check back later if nobody else can help.

If price is not of concern, go for a Mac Pro. In fact, any higher-end Mac should work, knowing that they are often used for this kind of profession.

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.

@sndfreQ That sound like a small Beowulf cluster to me, and I think that if you shop wisely, you could build a PC-based cluster with more processing power for less than a Mac Pro.

@jerv yeah that is likely going to be the case any way you slice it. Mac clusters on the whole cost more because of the “Apple Tax” however the cost differential on a single Mac wouldn’t be that off. That is, the way most do the purchase on the Mac Pro is to buy only a fast core without adding accessories, then buying all the third party add-ons (memory, storage, cards, interfaces, etc.)

That’s how I’ve always done it anyways…

@sndfreQ You can make a 4-node Apple cluster for under $1300? (Actually, “Microwulf” was $1256 in 2007) I am not sure what the Mac Pro can do exactly, but I am fairly confident that it’s less than 26GFLOPs.