I didn't realize slimebeast wanted to have a 2nd PC, which of course there is no reason to put XP on an new PC.
Anyways, it's all comes down to priorities and available money. I tend to follow the don't upgrade until you have to philosophy. Obviously if downtime at the wrong time is a concern, then you should upgrade beforehand. I also doubt those W7 communities are going to disapear in 1-2 years.
It's not about them disappearing entirely, it's activity. When everyone is switching over and encountering similar issues, you're going to have 10 people working to find solutions. 2 years down the road, most of those people solved their issues so they're no longer working with you so much as you just searching for wherever the solution ended up being. And, if there isn't a solution, the communities will be less active than they likely once were so it's harder to get people to help.
Compare it to video games. The best time to join an online community for a game is right after it launches because that's when everyone is most active. You can always join later, but people tend to leave them behind after a certain amount of time has passed.