I have a 40GB

TV that hadn't been used in over a year, so I decided to crack it open. I replaced the wifi card with the Broadcom Crystal HD BCM970015 PCIe Mini card, which enables 1080p playback and codec support for just about everything out there. I installed XBMC and Crystalbuntu, which is a build of Linux that some folks on the XBMC forums created specifically for use on the

TV. This absolutely turned my

TV into a new machine. It will play anything I throw at it with nary a hiccup. The drawback is that you lose wifi, so you either need a wired connection or a wireless bridge (e.g., Airport Express).
Plex doesn't run on the original

TV, as it runs on an older version of OS X than the software supports. You can run either Boxee or XBMC, but only XBMC supports hardware decoding using the Crystal HD card. And honestly, without the upgraded hardware decoding of the Crystal HD, the thing is fairly useless. I tried running Boxee with mine without the card, and it choked on almost everything. You can plain forget 1080p and .mkv files, and most 720p material stutters.
For me, the $50 for the Crystal HD card was well worth it, since I already had the

TV. If you're getting a good deal on a used one it might be worth your while to upgrade it. If you're already spending $150+ just for the

TV, however, you might be better served looking for something else.