Whats it worth
I remember when you switched from the RR to the 340 Dart Sport. Was this your inlaw's 340?
It is really tough to say what the car would actually sell for today. Obviously the market has really changed due to the economic crisis.
If the car is accident free, paint is decent and the mechanical end is very good (which as you said - appears to be the case), it all comes down to someone falling in love with your car.
Two years ago things were much easier to sell; today is tough.
So my guess is ...around $5500 - 6,000 ... but the caveat is finding the right buyer.
There may also be someone willing to spend more for your car ... You could always Ebay the car and roll the dice but I have found that many cars on Ebay never actually change hands...the deals are not often completed.
If you wait until Carlisle you obviously have your best chance there and likely your best price / offer there.
Whatever you decide with it...all the best!
Stew