Main difference are the ability to upscale as well as HDMI output. The 29xx is technically a model down from the 2900, being a successor to the 2200 in reality (not that that is a bad thing). You would want to look at the 39xx as successors to the 2900. Yes, those model numbers came out in successive generations, adding various features to each one.
$350 sounds about right for the 2900 these days, if it has original box, manual and remote. It is an excellent player both audio and video wise, however there are newer upscaling players that can be had for less that are as good or better (
www.oppodigital.com ). Another thing to consider at your price point would be either an HD-DVD or Blu-ray player since they can all upscale your standard DVDs too.
What exactly is the priority? Picture quality? Sound quality? Balance of both?