I think that is a question that photographers will be asking themselves for most of their photographic lives. Sometimes I wonder if I will ever be able to find the right combination of lenses and cameras to shoot in the conditions I want to be able to shoot in. During the spring and summer, as long as it isn't too late in the evening, I find that the D300 and other crop sensor cameras are more than enough to get the job done. Then the fall and winter months come, and I just know I'm going to miss shots because I have to bump up the ISO too high, and I start to loose the colour and image quality I want. When I run into situations where the light is too low like that, I long for a full frame camera and a set of f2.8 lenses. I may be getting a f2.8 lens in near future, but a full frame camera, may be a ways off yet!
Right now I am hobbyist photographer, which means that high end lenses and are far beyond what I can justify owning. Sure I could buy a 500mm F4 VR, but I'd also have to skip buy gas for half the year to help make up for it. Even if I could afford a lens like the 500mm F4, I'm not sure I'd want to carry it around, I think I'd have to buy a cart to move the thing around all day! So from a weight standpoint crop sensor cameras can be nice to have around. Although crop sensors do not magnify the image, as some wrongly believe, it does limit the field of view, which gives an appearance of greater reach. Hopefully within the next year or so we'll have crop sensor cameras that match or beat the current full frame cameras anyway. From that perspective it makes it a lot easier for photographers with crop sensor camera to just wait and learn to be satisfied with our equipment, if that is even possible. ;-)