If you're in a situation where you need a backup body, it's either keep the D2x or sell it and purchase a D300, which would probably cost about $2200 with battery pack and additional EN-EL4a battery, to make it comparable in handling with the D3. Frankly, that probably is a more cost effective process long term, since you'll be able to stay on the front end of the technology curve for not too much additional money. It also provides more compatibility with the D3, due to similarities in LCD, Live View, AF, etc. Also, you'll have the flexibility of a lighter weight body for those times when that's important.
The only reason I would keep the D2x is for the ruggedness of body and the higher frame rate in cropped mode, but I don't think that's really going to be an issue with the capabilities of the D300. I had a similar dilemma when I upgraded from the D1x. I kept it around through three upgrades and when I finally sold it I received less than 1/10 of it's original purchase price. Unless you're sure you'll use it, consider other options, but if you do decide to part with it, don't trade it in. Sell it to someone on these forums or on Craigslist, as I did with my D1x. You'll receive a more fair price and the satisfaction of knowing it's being used by someone who really will appreciate it. Good Luck!
OldPhotos "If everyone possesses some measure of this intangible quality called creativity, photography is unprecedented as an outlet for its expression." - Ansel Adams