Ive got a white 2.0 macbook and I love it. I really think the performance difference between the Core Duo and the Core 2 Duo's will be marginal unless you're really taxing your computer with heavy use. For standard day to day stuff, and for the normal heavy use a dual 2 gig Macbook has all the power youd ever need. I highly doubt Apple will lower the price when the Core 2 Duo comes out. Its already considered an upgrade and the macbook is already priced so well. I think the most likely thing youd see them do is add to the specs to differentiate between the models a little more. say,60 gig drive for the white 1.8, 80 gig drive for the white 2.0 and a 100 gig drive for the black. That way your getting a little more value for your money. maybe a small price drop for the black model. But who knows...
The screen is small if its your primary screen and if you use it as a desktop for day to day use. My macbook sits on my desk %90 of the time and im getting tired of the small screen. But, with the price of LCD's so low now its not a big deal to go out and get a 17 or 19 for $200 or less. With a larger external screen, and an external mouse the Macbook is so much more versatile than an I Mac.
As far as prices for a used black macbook, I have to think they would be pretty close to a used white 2 gig macbook. The black was a rip off from the start (no offence to anyone who bought one) $200 for a paintjob is not money well spent unless you have extra money to spend. If your buying a used computer its likely because you dont have excess cash to burn so your trying to be smart. Paying more for a black computer is not money well spent when your trying to conserve. This is why im saying a black macbook isnt worth more used than a white one of equal spec. Furthermore, if your concerned about screen realestate youd better to not pay a premium for the black macbook, get a used white one and spend the extra on a monitor.
As a Macbook owner its in my best interests to suggest a high resale value for what your looking at buying, but that may not be realistic, so the truth is better. I think a 2 gig macbook in black or white, with an 80 gig drive and 512 of ram is worth somewhere around $1400. If it has 1 gig of ram id say $1450-$1500 TOPS but no higher. That saves you the tax and a couple hundred bucks. If the guy is charging more for the black model id suggest you go shopping on the forum and find a white one for less. If hes asking the same or less than what other white ones sell for, buy it. You wont regret it... its a great computer. On your desk or in your bag its great.
I hope this helps