There is a fire sale going on right now at Ebay. You can get a like new, low click D2X for less than a D300. I will wait to sell any of my 3 extra bodies and have sold the D300 instead.
I have a D3, a D300 with vertical grip/battery pack, and 3 D2Xs.
A used D2X makes a better backup than the D300 because. 1. Lower price 2. Better build 3. Made in Japan for Professional Photographers. 4. Faster focus and frame rate. 5. Sharper pictures 6. Crop factor for telephoto shots works really good. 7. Native ISO 100 8. Can use my old SB28 flash better. 9. Uses same superior EN-EL4 batteries as D3. 10. Single charger fits in bag. 11. More compatable workflow.