Both Danny and Kort have said it....

The very likely first "non consumable" part to go will be the shutter...and as Danny says, they are rated in the 100,000 click range.  So, barring any bad luck, you should be good for another 4 or 5 years.
If you do require any major repair....forget it....you will be able to buy a newer, better model for less than what your D100 cost you.

"Operational" life for these cams is usually greater than "market" life....in other words you will be more likely to upgrade to a fancier version before your old one gives up the ghost.....

And yes, the D300 is very very nice....I think it is time you put one in your hands to see how things have changed in the last 4 years. 
BTW, 4 years is a life time in today's technological advancements.

In youth, we learn....with age, we understand.