your onto a winner
Exactly my thoughts on your situation. The thing about you not using all the cores is that its a great multi-tasking machine an 8 core machine is now taking full advantage of apples software running 64 bit and apple are leading the way in software development and everyone is catching up even adobe!!! lol, plus you will run more programs more smoothly here is an example this is the same as my mac pro and it is 2 years old, some guy showing off what it can do (not even the nahelem) :
http://www.youtube.com/watch?v=mXq5gsUS_4s
So in other words you will find browsing, watching video, itunes, iphoto, encoding video and playing games a breeze simultaneously! like i can encode 1080 video while gaming without dragging the experience down significantly! plus you have to remember that if your looking toward the future then hardware is always way ahead of software, 64 bit has been around for 10 years or more on the mac and only just being put into practise and worthwhile for general mac users.
In terms of the processors you have server grade processors in a desktop, they are designed for high usage and longevity. The mac pro is not like an imac, the original mac pro quad 2.66 is still extremely fast and is 5 years old people get carried away with benchmarks etc, and technology moves so fast, but this is an investment that will pay, it will last you at least 7 years of service without you thinking of upgrading the processors. But its the quantity and the quality of these processors! they have enough power to literally run anything! they are not designed like normal desktop chips say the core2 or i range, benchmarks may be similar but when you get down to doing some serious multitasking the xeon chips are the top trump everytime.
This is why pro's use them not iMacs, the core and i range is designed to have a shelf life, so intel makes more money the server grade chips do not sell like hot cakes unlike the desktop chips, and are not refreshed as quickly. If they did then companies wouldnt pay the premium of a high end server because it would be trumped and need replacing quicker. Mac pros are not as consumable like the more affordable iMacs, companies have to make money and upgrading them incrementally helps do this.
In terms of replacing the processors i dont think youl need to worry about this, 3 years! I think you can upgrade them but this will set you back nearly as much as the computer i seem to remember being curious about mine and finding it would cost me £1200 for 2x 3ghz quad core xeons. This is why they are so expensive they are designed for the future in mind. Professionals buy these and only get rid of them when they are literally on their last legs if that ever happens and in terms of them not being powerful enough, gaming is only going to get better, games will soon take advantage of more cores so even tho the computer is still old, in 5 years say, it will still feel fast. The games will be written to take advantage of the multiple cores! Pentium 4 chips are still good as dual or quads if the system is built with plenty of ram, just because the games arnt optimised for the multiple CPUs.
Mac pros are apples top of the line machine and hence the price and why they are the least popular computer apple supplies. But if you think you will change your computer every 3 years then a pro will survive at least 2 of these lifetimes if not 3, and at the end of the day it has the latest slots for expandability so you can add future tech anyway, like USB3 etc.
And on the graphics card front like i said earlier the future looks bright in terms of gaming, ipad iphone, steve seems to be coming round to the idea and with the best available gaming community/supplier steam now coming to mac, i see a new war on the gaming front. Manufactures will start developing cards because gaming is now viable!
We have seen in the last 3 years a huge step forward in the graphics development for mac because of bootcamp! also with the new open cl apple is trying to take power from the GPU (graphics) and so need faster cards so more and more powerful cards will become available, and i only see it growing! A mac pro is just like a pc you can chuck any component in it like any pci express card as long as it has mac software if its a music device say or if its an expandable card like firewire or usb there is no problem, same with the hard drives any will go in as long as they are SATA or SATAII.
Graphics cards still have apples firmware on them tho so unfortunately the graphics will be more expensive but as soon as third parties start making them and being competitive i see the price coming down dramatically!
hope this helps
tom