I should stress not to skimp on your case and power supply. Get a case that’s roomy and easy to work inside, with adequate cooling (more-than-adequate cooling; you never knew when you’ll have a heat wave).
We obviously have different definitions of incremental. I just added a new HD and in January I am planning on purchasing whatever the hottest graphics card is and adding it in. If I was changing 6 or more devices, I would consider that a complete makeover. I don’t have XP and don’t know when or if I will switch to it, but I am not really surprised that it wants reactivated after 6 devices are changed.