The installation on one computer isn't true, thankfully. What is true is that you have an unique cd-key. That is used to create an unique account at Steam. You can install your copy on as many PC's as you 'want'. But you can only play at one at a time, because Steam checks with copy/unique key is being played, also single player.

So installing now, buying a new computer tomorrow, again next week and again next month isn't a problem. Well, not for Steam anyway.

Have fun.