While there is no doubt that computing is going increasingly mobile, I would not like to be without at least one desktop pc in the house as part of a home network. I have a large music and photo collection which is stored on a NAS device with a mirrored RAID array. This effectivly acts as a media server wirelessly sending to the rest of the network as well as over the internet.
Having a robust infrastructure gives you options that can help with security and back up and while it is nice to have laptops and tablets, on their own they do not have the performance for power hungry applications.
Since you already have a decent laptop, I would go for a i5 or i7 processor, loads of memory and Windows 7 (64bit) in a tower and perhaps an android tablet as well if the budget will stretch to it.