Yeah, milage varies sure, but look at what has been available in the past 3 or 4 years. A little over 3 years ago the first intel MacPro was released. A quad 2.66 was about $2400. Just a few months later Intel released socket compatible quad core chips. If you acted quick you could sell the X5150 duals for $600ea and buy the X5355 for $800ea making the upgrade to an octad only $400 in total. Roughly speaking of course. Between then and now there have been no compelling releases so my 2006 octad is pretty close to the same performance as 2.26 octad costing almost the identical dollar amount part for part / spec for spec.
So really regardless of usage patterns the 2006 Mac Pro has held it's value almost exactly 100% for three years running. I don't see Apple going for the 6 or 8 core chips and I don't see Intel offering 4 or 5 GHz xeons within the next 2 years so I guess the Mac Pro 2006 1,1 will still be nearly at 100% of it's original purchase value at it's 5-year mark (assuming of course you upgraded the procs to quad cores somewhere along the line). For years 5 and 6 I assume the 3GHz X5365 chips will be available for about $200 and if purchased will bring the total longevity of the 2006 to 5 or possibly 6 years at nearly 100% of function as compared with new Apple MP offerings at the same (or higher!) price.
Probably sometime within those last two years it'll be in the general user's best interests to keep an eye open for new systems but if we're comparing Apples to Apples then so far anyway, it looks like they hold their value at ~100% for ~5 years no matter the usage pattern.
