Don't worry about it being heavy, you'll get used to it. I have tiny hands and I love heavier cameras, which was one of the reasons I bought the D70S and sold my D40.
The D90 is an awesome camera, much better than the XSI or the Sony Alpha. The 18-105 is also a good lens, but if you're going to sell it I recommend saving up for a Sigma 18-50 F2.8. It has near professional quality at only $100-200 more than the 18-105. Be thankful, its a great camera to begin with and a great camera to grow into. Unless you were willing to buy a D200 or D300, I couldn't recommend a better camera.