there is a bad apple in very truck load. i think this sums it up pretty well. however, if you are not the "lucky one" to get that bad apple, you shall be set for a long time. however, the question remains, are you going to be willing to use a 6 year old system in 2011? answer that question keeping in mind that the gap in technology between 2005 and 2011 will be greater than that between 1999 and 2005.
speaking of which i have an earlly 2000 desktop..Dell. and after some upgrades, more ram, new and faster hdd, nvidia 5700 128mb, it still is being used every day and can even play some somewhat recent stuff, however i doubt that my late 2004 Uniwill will be able to do that by 2011 (that is why i am ditching it in 2007). although this is an example with pc computers (and desky vs lappy), simular is true for the mac world.
also whoever said that macs actually get faster with software upgrades, that is only true if you keep it in perspective. if that were to always be true, a 133mhz powerpc will be running circles in os x vs the same powerpc running system 7. even more so, g3 2nd gen imacs (the ones with the cd slot, not tray) would run OS X faster than OS 9. that is simply not true (yes, i speak from experience). perhaps if you add more ram to the system (say 256mb vs the 128) than it will, but here you're forced tp upgrade hardware. keep in mind that what you said is only true on the latest gen systems. lets say you run os 9.7 on the 1.25ghz ibook, and then run 10.3 on the same config, clearly the 10.3 will be faster .
as for your opposite trend on the x86 systems, also not very true. you clearly named windows. let me remind you that the NT platform (xp) is faster (assuming the system is recent enough with plenty of ram) than Win98 or even Me (9x). They promise the longhorn kernel will further improve this speed, but, it will require quite high hardware to start off with in the first place. also, in linux, kernel 2.4 was slower than 2.6 on the same hardware (speak from experience on this one). the bottom line being, there is only so much the software can improve the speed and efficiency of the hardware untill it is just simply time to buy a new system.