I'm "temporarily" keeping my '05 D-cab, probably wait 'til '09's come out, now. But I can tell you that dealer trade-in for '05's in the Northeast range from $17-20k regularly. I got a real offer for $22k on mine but that was on an in stock '08 CrewMax that had been on the lot for 3 months. My '05 only has 24k miles on it, so it'd pull the higher range for trade values.
$20k trade on an '06 seems reasonable, try to bump them another $1000 and call it a day. Punch in all your options at KBB,
edmunds and NADA. You'll get three Trade Values, take the average of the three and add a grand or two and use that as your starting point. That or accept their trade offer, but get them to throw in a few dealer installed options you want/need. Good luck. BTW - shop around other dealers, know what invoice is on the new truck and what you're willing to pay for it, and what you're willing to accept on your '06 for trade.