For about $4,000, you can now buy a Mac Pro fitted with a pair of 3 GHz Clovertown Xeon CPUs, creating an eight core OS X workstation and the meanest Intel Mac in Apple history. It's precisely the same box as the formerly-meanest Intel Mac in history, the four core Mac Pro, of which breed a well used and well appreciated exemplar sits at my right elbow, driving a 30-inch Cinema display.
The high initial cost of Intel's quad-core Xeon CPU, that price being buoyed by AMD's delayed arrival in that space, precluded Apple's usual tactic of transforming Mac Pro from four cores to eight overnight. I project that in Q3, when price pressure from AMD does come into play, Intel quad-core Xeon prices will fall to levels now occupied by dual-core parts, and the dual-core parts will begin to roll off the roadmap. As this happens, Apple will probably make the $3,298 3 GHz Mac Pro base model an eight-core machine. I'd be very surprised if Apple had a date set for that yet. Intel will squeeze maximum margins out of its quad core parts for as long as it has the field to itself (and it should).
Intel recently announced quad core CPUs with reduced power requirements, and the close timing of Apple's announcement might have caused some confusion. The 3 GHz Clovertown CPUs are not reduced-power devices compared to the dual-core Woodcrest. In a workstation configuration that includes 8+ GB of RAM, a dual-slot graphics accelerator, multiple 750 GB hard drives and Fibre Channel/10GbE/Infiniband bus adapters, a wattage drop affecting the CPUs alone would hardly be noticed. The reduced-power quad core CPUs will find their way into iMac as soon as Apple can lay hands on them.
There is not yet an eight core option for Xserve; the 3 GHz dual core Xeon is still Xserve's top CTO processor option. My hunch is that Xserve is in engineering right now and will re-emerge as a native eight core server. This will be part of a program that will see Apple gunning for enterprise business again now that the dust has settled on the consumer side.
What about an after-market upgrade to take your quad-core Mac Pro to octo? There is none in the offing from Apple, and it's a none-too-appealing option to tackle on your own at the moment. A manual chip swap to take an existing Mac Pro to an eight-core configuration will set you back at least $2K, and that's just for parts. As I say, that will get a lot cheaper.
I believe that $699 is the right price for the Clovertown CTO option. I expect--and this is based solely on experience with non-Mac x86 systems as I don't have an eight-way Mac Pro--that going from four to eight cores will reduce the time to completion for compute-intensive, non- or semi-interactive, threaded workstation task units (e.g. render, simulate, compile/link, analyze, transform, filter) by ten to thirty percent without code changes. (Sorry for the mumbo-jumbo. I'm putting a fair bit of energy into developing criteria for meaningful performance characterization lately)
Would I shell out the extra $699 for the extra four cores? When I look at the Mac Pro I use, I'm not wishing that it had more cores. A maximum of 16 GB of memory is very tight for an 8-way, 64-bit box. When I dream about the Mac Pro after this one, I see standards-based system management, larger memory capacity, better power efficiency and faster connectivity. Mac Pro feels just about balanced to me as a four core machine, but you can be sure that I don't work this Mac Pro the way you work yours. As a developer, I'm also living in Leopard, which, as any enterprise OS does, inevitably changes the way you look at x86 hardware. Leopard scales so bloody beautifully.