Kind of surprised no one has mentioned Plex yet. Install the server app on your existing iMac, and the client app on any computers you would use to watch media. The server app will serve media to any of the clients in the background, allowing you to use the iMac for other tasks. The server can stream MANY file types, including M2TS.
As far as storage solutions, look at the OWC Mercury Elite Pro Qx2.
$260 for the empty box, and add hard drives to your liking (must be same size). With 3TB Seagates selling for $125 each, you could have a 12TB (9 usable TBs if arranged in a RAID 5) storage solution for around $750. If you have spare drives sitting around, your costs will come down.
Personally, I am using a Mac mini to run plex server (and client, because the mini is plugged in to the tv). My media is stored on a 12TB Synology NAS (a little more expensive than the OWC listed above, as it hooks up via ethernet instead of usb/firewire). I run the plex client app on the mini, an upstairs iMac, a jailbroken Apple TV 2, two iPhones and two iPads. Plex also works with the andriod platform, windows phones, and Raspberry Pi.