<p>I kept my D70 for sentimental reasons when I bought my D300 ages ago. I thought I could use it as backup, but also because I wanted to place it on the shelf, look at it, and let the thoughts flow...<br>
Have I needed it for backup? No.<br>
Have I really looked at it, and been sentimental? Well... Maybe...<br>
I tried to sell it dirt cheap this fall. No one wanted it...<br>
I sold the D300 when I bought a D700 a year ago. I still got some money for that, and I have not regretted one minute for selling it.<br>
Lesson learned. Take a photo of your D300, and look at that image if you want to be sentimental. You will probably not need your D300 as a backup, and as Shun and others say, get some money for it while you still can. </p>