I came across of an interesting articles: "How to make Windows XP last for the next seven years (Vista, schmista. Follow our tips for keeping your XP setup humming happily for a long, long time)" by ComputerWorld (full article here). I do recommend using Windows XP over Vista, although Vista looks great, it's really a resource hunger and eat on your wallet, alive. Think about that, nowadays you pay for an Intel Core 2 Duo CPU and 2GB of ram, and only got some performance like an Intel P4 with same Mhz and 512MB ram. And don't forget that, you already close to or reach the upgrade limitation, esp if it's a laptop. Usually, I need a lot of memory to run a lot of application at same time. Therefore, I do not wish the OS took too much of that from my machine.
Anyway, let's keep our Windows XP SP2 (or SP3) for at least another five years.