Following up on the choice of CPU question...
I have both a low power machine i3 I run as a simple HTPC client with HD3000 and it works great for that in our basement TV room.
I have another mixed use machine with a 2500K that I use for ripping and transcoding my dvd collection. I've been working on a collection of DVD's, CD's etc each weekend for a year or so now. I will say that if you are planning on doing this with your server (I'm assuming you're planning on serving media to other machines) then the 2500K and some extra RAM is a great investment. There is a very noticeable reduction in the time it takes to reencode each movie (especially if you get into Blu-ray rips which are huge) and you're future self will thank you for the time savings. It is still probably a bit pricey to spend that much if you are ONLY planning on getting this benefit out of it, but you can weigh whether the time/hassle is worth the extra cost, especially if you might transplant the 2500K into a more main use machine down the road.
If you're just storing and serving up files from the server then you could even go headless with the machine and remote in to manage it.