It's not really a question of waiting or not --- You'll have to wait the same amount of time whether you buy it retail (self-install) or buy it pre-installed on a new PC.
The real decision is whether you want to save some cash but risk having all kinds of compatibility problems by installing it yourself on to an existing machine, or spend quite a bit more by getting a new machine with Vista pre-installed and working properly.
Since Vista has some very specific hardware requirements, I personally would go for the pre-installed new PC option. That's if I was keen on having Vista, which I'm not. XP does all I need, and more. Vista will only become part of my life when my present PC expires and all the new ones have Vista whether you want it or not.
