XP Pro - are you sure? Unless you're on a domain or use any of its other features, you can save some cash by going for XP Home instead. But I applaud your decision to stay with XP for the time being, you can always install Vista later on if you want.
You have the components right, and your motherboard supports socket 775 processors.
When you've got your current PC upgraded with those parts, your system is going to fly - enjoy!