I am thinking of opting for a 256gb flash drive, and connect my 6TB external hard drive via thunderbolt and use the computer as a server for all my multimedia needs. Ideally, I would run 2 or 3 screens on this mac pro.
Since the thread seems to have descended into a flame war, lets try some common sensical musings
1. Let's assume that you want a Mac. Take it as read that you can get something from Dell that looks like a refrigerator (maybe a refrigerator with wings and fluffy dice) with twice the raw speed for half the price, and your choice of Windows or Linux - or a Hackintosh that will give you a weekend of nerdy entertainment every time Apple release a software update.
2. It's your money. Would someone buy a sports car even if you never go near a race track, or a powerful saloon with a top speed of 3 x the maximum speed limit of any road you are likely to drive on? Hell, yes.
3. The nMP is
clearly optimised for users running software that can use (or is likely to be updated to use) OpenCL to make use of the GPUs for number crunching. Realistically, that means video, 3d and graphics software. As for the CPU, Xeons aren't necessarily better than i7s if you're not doing workstation-y things. It will almost certainly be very, very nippy for the sort of general uses you are discussing - but the improvement c.f. an i7-based iMac or Mac Mini may not be proportionate to the price difference.
4. Killer features - you mentioned 3 screens. If you really want to run 3 screens of your choice without using up all of your thunderbolt ports and/or needing at least 2 of them to be over-priced Apple Thunderbolt Displays, then that is almost a Unique Selling Point for the nMP. Next best choice for that is probably a Retina MacBook Pro.
5. Alternatives - a fully tricked-out iMac will cost less and throw in a rather nice display, and you can have 2 screens easily (3 = possible within constraints). What puts me off is the non-upgradability of the HD/SSD - with affordable 1TB SSDs likely to appear within the useful life of the machine. (That's assuming that the nMP SSD is going to be upgradeable). Mac Mini is relatively cheap, but lacks discrete graphics, and there are constraints on multiple monitors, but at least you can unofficially upgrade the SSD. Its also overdue for an update (which I assume will happen once the Mac Pro is safely launched).
6. Longevity. The days of new computers being hopelessly outclassed after 18 months seem to be over. I sidelined my 2006 Mac Pro in favour of a laptop a couple of years back, but quite honestly, if I threw in some more RAM and an SSD it would still be a perfectly good computer - and old kit can always find new jobs as a server or hand-me-down. 10 years is probably pushing it, though.
However, the new Mac Pro is a very "brave decision" for Apple. I think they've correctly read the writing on the wall for the "big box 'o slots" workstation - but it could still flop, and is very dependent on the success of Thunderbolt. If it fails, then you could find yourself reliant on eBay for upgrades and spares sooner than you'd like. If you're looking for a 6-7 year life span I'd wait and see, and maybe wait for the first update.
On the other hand
. mmmm
. shiny!