I've had my D3100 for nearly 2 years now. I was considering changing to a higher spec body this year, as I wasn't happy with a few of the images I was taking. However, I took some advice from people on here and elsewhere, and went for a new lens instead.
£370 later, my pictures are now much sharper and clearer, more so than I had imagined they would be. So, don't just buy a new body because it sounds better. Invest in a good quality lens first. Once you've got the hang of that, then you can start looking at upgrading the body...
Chris