Advances in desktop CPU speed have been minimal over the past few years. If I could get a reasonably priced PC that's 50% faster than my two-year-old system, I'd buy it today. Instead, the best performance gain for me is a PCIe SSD.
energy efficiency since i purchased my computer has improved greatly, but i have no reason for a more efficient CPU, its still performing fast enough for all desktop applications and has no battery.
I've upgraded them both to SSD, added USB3 via PCIe [1], and bumped the memory up to 32 GB in the 2009. I mistimed [2] the memory updates on the 2008 so only got that up to 12 GB or 18 GB (I don't remember which).
Neither of these machines shows any signs of needing replacement.
If I used them for gaming perhaps things would be different. Now that I think about it, all of my purchases of machines to run Windows over the last 15 years were because I wanted better gaming performance.
[1] http://eshop.macsales.com/item/Sonnet%20Technologies/USB34PM...
[2] Memory prices often follow a U shaped curve, starting out high when a new speed of memory is introduced, then falling as that new speed becomes mainstream. Then something faster and incompatible comes out, and as that becomes mainstream production goes down on the older item, and prices rise.
For example, current prices at macsales.com of 4x8GB in various technologies for Mac Pro:
$290 1866 MHz DDR3 ECC Used in 2013 Mac Pro
$200 1333 MHz DDR3 ECC Used in 2010-2012 Mac Pro
$195 1066 MHz DDR3 ECC Used in 2009 Mac Pro
$780 800 MHz DDR2 ECC Used in 2008 Mac ProThe trick is to buy models that you can replace battery, memory and HD.
Already that brings the laptop a very long life, before the motherboard or screen eventually die. Or even the memory limit doesn't help anylonger.
But even in 2016, 4GB - 8GB are still good enough if one isn't running VMs or development environments like InteliJ.
Interestingly, before this 6-year-old PC, I'd been upgrading every 2 years, but I haven't felt the performance pinch since (okay, some games were a bit pinchy with what used to be a good card, but otherwise no)