Will it use an 8-core/16-thread Nehalem Mac Pro to its full capacity?
Depends on how many threads a specific task will spawn in SL. If it's less than 16 threads on an 8-core machine, the answer is 'no'.
Personally I would not expect many OS tasks to even exist that can easily be spread onto 16+ threads.
But SL will undoubtedly run much better than Leopard which in most cases uses just one thread for tasks.
In reference to SL, Yes, some aspects will improve due to the ability to multi thread.
But it will depend on specifics though. Particularly where user input is involved for the completion of a task.
2.)
For example it is quite possible that the highest high-end graphics card in 3 years will require a PCI speed (or successor bus technology) not even the current Mac Pros can provide.
Possible, but the next three years does look to continue with PCIe. Even PCIe 1.0 x16 slots (operational) aren't hitting the bandwidth limit with existing graphics cards, and we're already getting PCIe 2.0 slots.
So it's going to be around a bit yet (3yrs is certainly reasonable).
We are already seeing this on first generation Mac Pros (from 3 years ago) which cannot use current high-end graphics cards as their PCI bus speed is too slow. Or if they can use it, it will not run as fast as on the current Mac Pros.
It's the issue of the EFI firmware is 32 bit in those machines. They can't work with 64 bit EFI cards.
The PCIe bus isn't actually saturated. Either way, it's a limitation for their owners
apple:'s not offered a fix), but not a result of the PCIe interface.
Or the current Apple RAID card is not compatible with first gen Mac Pros, yet it is the only RAID card from Apple that is well bootable under Mac OS and BootCamp.
So if you need a BootCamp capable RAID card, then today a 3 year old Mac Pro will give you less optimal choices.
Apple's RAID card boots OS X, but won't even work under Windows, let alone boot it. It's also slow, especially for the money. No RAID 6 either, and for $700USD, it should.
Simply put, an overpriced piece of junk.
For this, you have to go with a 3rd party card. To actually use Boot Camp (both OS's on the same
single drive, not the array), you have to use CalDigit's RAID card (also junk IMO). It can't boot both from the array. To boot, it's one or the other.
The next solution, is to use separate OS drives, separate arrays, or in a pinch, partition the array (same physical drives, but separate logical partition for each OS). There are 3rd party cards that work with both OS X and Windows, and even a few that can boot both. They happen to be better cards (more than a few threads on this
).
3.)
As was said before, once you open a box and start up a new Mac, returning it will incur a 'restocking fee'. You will have to pay somewhat for 'trying things out'.
So no, you will not get a full refund.
But you will get a huge portion of your investment back if you return it within a few days. But don't wait too long as there is a time limit.
14 days from date of reciept of the system (
Apple's Return Policy).
In case you were to get the high-end 8-core 2.93 Mac Pro and should find it too slow, then you're plain out of luck. You then can only buy another non-Apple product or wait for faster CPUs in future Mac Pros.
Or in other words, if you need a new Mac now, it might be worth getting a 2.93GHz 8-core Mac Pro (refurbished if you can't afford it new) just so you know, that if it's still too slow, you will just have to learn to live with it as it is as good as it gets.
Or build a farm, assuming funds are a non issue.
4.)
This will again depend on how well your applications of choice can use multiple threads.
If you use applications that are mostly single-threaded, or can use multiple threads only for certain, few tasks (Photoshop CS4 comes to mind), then you are much better off with a 4-core 2.93 Mac Pro as Photoshop will likely use only one thread anyway, and in that case it's better that thread runs at 2.93GHz than 2.26.
But eventually even Photoshop will become more and more multi-threaded. Perhaps not CS5, but some future version will run faster on an 8-core 2.26 Mac Pro than a 4-core 2.93 - if it can spread its computation evenly onto all 16 available threads.
But this will still depend on the Photoshop action you use.
Today, some 3D rendering applications can already use all 16 threads and hence are faster on an 8-core 2.26GHz machine than a 4-core 2.93GHz machine.
In either case it is expected that in future more and more applications will be multi-threaded and hence even though today a 4-core 2.93GHz machine will overall likely seem faster, the 8-core 2.26GHz machine should ultimately become faster overall.
This particularly valid with 3rd party applications. Not everything is capable of operating as multi threaded though. I don't see word processing getting much faster, for example.
Applications such as rendering and scientific/simulation,... however, can benefit, but may require rewritting the application to take advantage of this capability, assuming it's not already been done. Given some of the various threads, not everything that could benefit has been, unfortunately.
Such redevelopement isn't on Apple's schedule either, so time frame to deployment is variable, depending on the software developer's product development cycles, available resources, etc. Perhaps in terms of years.
5.)
Unfortunately not. This was a feature in the previous generation Mac Pros, but the current 4-core Mac Pro has neither the second (empty) CPU socket, nor the chipset required for a dual CPU setup.
However, the CPUs are now on a daughter board which can be removed. In theory it might be possible in future to swap a single CPU daughter board for a dual CPU card.
But Apple has not announced any such upgrade path, and it might never be made available officially.
And on top of that as far as I know no one has even tried to swap daughter boards just to see if this would work in theory.
It can be done, but the cost would be staggering.
Perhaps in time, a viable "parts" machine would surface, but would take time, patience, and not something to really count on as an upgrade path.