I believe that with Vista x64, quad-core processor and plenty of RAM, you have a good chance of not needing to upgrade for quite a long time.
Vista x64 is slightly more secure, slightly faster on modern hardware and can handle and can give applications more memory than the 32-bit version, which makes it more future-proof. It is also quite a bit faster with x64 applications, but those are still quite rare and tend to be rather specialized, although that will change with time. Since the x64 version of Vista has a slightly higher memory overhead, I don't see a point in using it with just 4 GB, which is why I have the x64 version with 8 GB installed. I love the fact that I can load a 1 GB picture into Photoshop and work with it without waiting for the hard-drive to stop swapping after every single operation.
In fact, my advice for building a new computer would be to choose the hardware that is later than the OS installed and is fully optimized for that operating system, load motherboard with as much RAM as it takes and then use it as long as the applications you need work smoothly and fast enough for comfort.