Short version: I drove a new RX350 home this evening.
Lexus' semi-annual sale ends Monday, and the dealership is closed tomorrow and Monday. They made me a good deal with very good financing, and I decided I couldn't pass it up. Since I owned the RX300 outright, I was able to get the RX350 for lower payments than I'd made on the RX300.
The new car has the premium plus option package, with the navigation system and 18-inch wheels, as well as the other stuff you'd expect from a well-equipped Lexus. The navigation system comes with a rear view camera that's on when the car's in reverse. I'm going to be making good use of it, since the car is 6 inches longer than the old one, necessitating precise positioning in the garage. It's the same color as the old one, pretty much (though it's called Savannah Gold Metallic, this time around).
The 3.5L engine has plenty of power, though the price of that is having to run premium fuel. It's even quieter than the old car (vakkotaur's comment was that "they started with quiet and took the noise out"), which came as a very pleasant surprise.
I'm going to have to take it back in next Saturday to have the XM receiver and trailer hitch installed. While I'm there, I'll remove the FT857 from the RX300, though I'm not going to install it in the RX350 anytime soon. I've already removed everything else I want from the RX300.
I wasn't expecting to buy the new vehicle this soon, but I'm happy I did.