your computer will never slow down. it's just going to take longer to do harder tasks.
Exactly. Ignoring the effects of internal wear and tear, the machine will not slow down. If you can avoid the temptation of upgrading all the apps and OS over the next five years, then you'll theoretically have a machine that proves right now it can manage all of your listed tasks. However, one of the great things about Apple is that generally their operating system upgrades are fine on older machines (to an extent), unlike their Windows brethren. Tiger was a bit of an anomaly in that it uses a fair chunk more resources than Panther and doesn't run much faster. Otherwise, most OS upgrades over the past few years have been better optimised for the hardware and therefore run even faster than before. With that reasoning, the current MacBook should be fine in five years, albeit not compared to the latest hardware at the time.