I think it depends on whether Vista is going to offer you something you really need. For me it doesn't so I would upgrade now. Besides which, there's no telling when Vista is actually going to arrive. It's already been pushed back more times than Max3D.
...Yet.
Even if you don't plan to rush out and get Vista, the market situation may be vastly different 1-2 years from now. Do you still intend to be using your 'new' PC at that point? If so, and there is no real reason to buy a new PC right away, you may want to hold of a few months buying right now, just so you can potentially gain extra *years* operation out of your new system.
Something to consider: vista uses a new driver model again. There's a real chance that an item like a motherboard or PCI adapter or something that you buy today won't have Vista drivers available. If that particular model gets phased out or replaced two months from now, there's a good chance that many of the motherboard manufacturers wouldn't exactly go out of their way to create Vista drivers for 'em -- many would rather just sell your their latest and greatest motherboard out on the market at that point, which of course will have the Vista drivers.
Even though I personally have no inclination at this point to buy Vista at all, it's still something to consider. No one knows what the market looks like a year from now, might as well play it safe.