You are considering Windows Vista, and wonder when to upgrade. You must be kidding me, right? If you ask me advices on this matter, I would answer: don't bother. There are several reasons for this, which I will explain in details below.
First, Windows 7 will be out really soon. As far as now, some of my friends have had their hands on the beta version of Windows 7, and the experience, according to them, is really great. One of them said that Windows 7 is like Vista without all of its hassles. If this pace is kept, Windows 7 will be out in 1 or 2 years max. Windows XP will be supported until then. Thus, why bother? Vista is not free, nor is Windows 7. If you want a new experience, I would say save money and wait for Windows 7. After all, isn't it what Microsoft is encouraging you? Their releasing schedule allows you to skip Vista totally.
Secondly, what would you gain from Windows Vista? Interface? Frankly, I have seen Windows XP that is more beautiful than Vista. Of course, it is NOT free to do so, but the prices of these software (StyleXP and other similar products) are WAY cheaper than Vista. Plus, they keep the weight how you use your PC. When you upgrade to Vista, EVERYTHING changes. Some of those changes serve no particular reasons. Some other changes worsen the situation. In the end, the only real gain is the interface, which, as I explained, can be done in XP with minimal cost and effort.
How about security? Sorry, but I would argue that Vista WEAKENS the security of Windows. Why, you ask? Because it treats its users like a fool. Imagine if you are asked something three times whenever you do it, soon enough you will consider that asking as a hassle. Then, you will say "yes" out of habit. This is particularly dangerous, since the protection layer for you is totally discarded by your habit. Thus, although it has many protections, Vista largely abuses these protection to a degree where these protections become useless. Meanwhile, in XP, you can install addition softwares (firewall, antivirus, adware/spyware detector, etc.) and achieve the same, if not higher, level of security. Of course, in Vista, you still needs these softwares, only less effective.
Furthermore, the cost to switch to Vista is pretty high. Installing Windows is a time consuming process, since you need to hunt down all drivers you need, plus you will be briefly expose to the Internet without protection before installing necessary softwares. Furthermore, Vista does thing differently from XP, a lot. You will need to re-learn how to use your computer for practically no reasons. Plus, there is some hassles that does not exists under XP. For example, in Vista, if you have only one account (which is how most personal computer is used) and you forget your password for that account, the only way to go is, ahem, reinstall the whole OS. In XP, at least you still can use the installation CD to activate the hidden administrator account, and change your password from there. In Vista, ONLY reinstalling OS (oh, there are other ways, but these ways are generally too difficult to do).
In the end, an user has no reason to upgrade to Vista. Again, it makes much more sense to wait for Windows 7. Oh, if you are too tired of XP, why not give GNU/Linux or BSD a try? They are very powerful and secure operating systems, plus they are gorgeous. Lastly, statistics show that learning GNU/Linux is easier than learning Vista, from a XP user's view point. Again, there is NO point upgrading to Vista.