It isn't just about how much care you have for the machine, it has also to do with what do you do with your computer.
Games usually are the ones that requiere more and more power, in case of a laptop this is bad because they usually ask for better video cards or more video memory. Think about the kind of games you want/like to play. A hardcore gamer will upgrade his/her computer every year, this will reduce the life of a laptop or even a desktop, it won't matter if it is a PC or a Mac.
Media files are also important, the best example here is the jump from TV quality to DVD quality to HD quality. If you have a digital camera with 4mp and you just shoot in JPEG, but suddenly you go to 12mp and RAW it will affect your machine's performance. Mac's in general age better in this case, all you have to do is add RAM, they may be slower but they get the job done. Windows seems to be faster, but as the files get bigger it crashes a lot.
The version of the programs you use are also important. Photoshop, for example, they release an upgrade every year, with more and more options. This new features usually require more power. I used Photoshop CS in my iBook and now i'm using CS2, it takes a little more to load, but the performance is similar, for the kind of things i do with it, i can't really say there is an important difference. From my experience with PCs this difference tends to be bigger.
Today i spoke with a guy who still uses one of those black PowerBooks with OS9, he has taken good care of it and it looks great, he doesn't need an upgrade, it still works great (for him). My brother's desktop used to be an AMD Athlon 2800, all that is left from the orginal configuration is the case, in about a year he had to upgrade in order to play the games he wanted to play. I got my iBook on March 2005 and it is still working great for me, i was thinking about changing it at the end of the year, but it still works for me, and it will be my primary machine for at least one year more.
I hope this gives you a different perspective to make your choice.