New camera or new lens (or both) to shoot better sports photos in low light?

I am the photographer for a soccer team and have taken some pretty good shots with my Nikon D50 using a Nikon AF Nikkor 70-300mm 1/4-5.6 G lens.  I'd like to improve the sharpness of my photos in bright sunshine, but more important, I'd like to be able to take decent photos on overcast days (or even indoors) throughout the range of my zoom lens.  I am not a professional, but a serious enthusiast.  I've narrowed my options down to the following:

1.  Keep my D50 and spend the big bucks on a 70-200mm f/2.8G VR II, so that if I ever get even more serious, I'll have a good lens that will work with a full frame camera.

2.  Keep my old zoom lens and buy a D600 (about the same cost as option 1)

My question is, which option will improve my photos the most noticeably?  Thanks in advance.