I was in the same position as you. D300 and ƒ2.8 lenses were a pain to carry around all the time and I almost stopped using them. But, I bought into the micro 4 thirds system ( Panasonic GF1 and lenses) It was so convenient to carry around I found myself taking it out all the time and using it.
It seemed to take over all my shooting time and I eventually sold my D300.
I wish I had kept it..
After a year or so I found myself hankering after a big DSLR again and bought a D200 ( can't afford another 300, yet).
I still use m43 ( now an Oly E-PL2 with 14mm lens and a Panny G2 with 14-45 and 45-200) IQ is very good but now I have the choice. Big camera and big lenses or small camera and small lenses. One or the other is always with me.
Get yourself an M43 camera and lens, get your interest in photography going again then you will start using the D300 again.
Worked for me...
Allan