First off, by "Is HD or Plasma better?" you mean "is LCD or Plasma better?" (they're both HD, just different technologies).
Okay, starting with the TVs - Sony is generally known for having the highest quality (and most expensive) HDTVs, but a pretty good midrange brand is Samsung, they have reasonably priced TVs (42" in the $700 range, which, compared to Sony's, is a godsend) with pretty good quality. Avoid off-brands like Element and Insignia, there's always something that's really NOT great about them.
Most TVs you'll find these days are LCD screens anyway, but in terms of which is better, Plasma or LCD, it's really up to preference. Plasma, I believe, produced an ever so slightly better picture, but they're also more expensive, and they tend to burn images into the screen if left on too long. LCDs are generally more reliable and don't burn images. You should probably get an LCD TV.
Computers: All modern computers should have at least 2GB of RAM to be able to run smoothly, but if you must downsize from this, 1GB is necessary to even function with today's software. Most PCs should come with 2GB of RAM. For hard disk memory I would recommend you stick with hard drives 120 GB and up, because this 80GB drive in my PC is KILLING ME. One more important thing to look for: processor should be at least Intel Core 2 Duo (no Intel Celeron or anything in that range).
That's all.