Depending on miles, reg vs quad and of course the year you could be as low as 25 or as high as 35 for a stock truck. I have seen some 04s and 05s with 30K plus go for 25K, so figure 20K on the odometer or better for the mid 20 price.
04s have smaller brakes and 4.10 gears, where as the 05s and 06s have 4.56s. The 06 has the newer Ram frame. 04s do not have a knock sensor. I also think, and this is opinion, the 06 interior is ugly compared to the 04/05. Other than that they are the same.
Another thing to consider, I think that prices will change once all the new Rams are off the lots. Right now you can still go to the dealer, well maybe not in your home town, but somewhere, and get a new 05 or 06. Once those are gone the used market may begin to change to more of a sellers market.