Wow...I am surprised nobody gave you more details on this. In a nutshell, the 3910 is basically the 5900 performance (with better video I might add) at less than half the cost. The 2900 is nowhere near the caliber of the 3910. Trust me, I own both!
here are some reading for you:
Secret's review:
http://www.hometheaterhifi.com/cgi-bin/shootout.cgi?function=search&articles=all#DenonDVD-3910%20DVI/HDMI
Follow to the review:
http://www.hometheaterhifi.com/volume_12_1/denon-dvd-3910-dvd-player-1-2005.html
Audioholics Review:
http://www.audioholics.com/productreviews/avhardware/DenonDVD3910Review1.php
Their player of the year:
http://www.audioholics.com/productreviews/avhardware/2004productawards.html
Finally, here are some of the awards the 3910 has garnered thus far...just a sample:
http://www.avsforum.com/avs-vb/showthread.php?postid=5234344#post5234344
Yes, the 2900 is a bulletproof performer, but the 3910 is at this moment the best player out there, lb for lb (or shall I say, $ for $)