The Qnap range are also worth checking out.
Excellent for what you plan to do – all our music, video and files are stored centrally and accessible from any machine on the network (desktop, laptops, mac mini as itunes/media machine and also readable direct by the DNLA equipped TV).
Choose an enclosure based on what you plan to do then add your own hard drive.
Might be worth going for a twin bay if you can afford it – at a later date you can add a second drive to expand storage or migrate to a larger drive plus have the option to run RAID in case of a drive failure.
CPU speed mostly affects file transfer on/off the device but won’t come into play when streaming HD.
Finally, I’d also look for one with a backup function. My Qnap is set to automatically back up when a USB drive is connected to the back port, which makes complete backups quite and simple.