Like I said in another comment, in my opinion, a 2006 (intro of a intel's c2d) computer with 8gigs of ram + an SSD was pretty much the last time your average programmer saw any appreciable gains on the desktop.
Things _have_ moved forward since then, especially if your are multi-thread/core sensitive. For everyone else, even at 8-10hours a day, I'm not sure it'd be considered a great investment for such incremental improvements.
Unfortunately in some (most ?) big enterprise we are still stuck with Windows XP (so 3.5gigs of RAM) and shitty hard drive that are further killed by the antivirus :(
RAM and i/o will certainly make the biggest difference for the average person or engineer. That and HID's and your display.
My computer before this one at its core was not the latest/greatest Mac Pro, but I had upgraded parts (RAM, video, HD) which made the biggest difference.
