Ive got a D5000. I have never used a D3000 but the only real differences as far as I can see is the D5000 does 12.3 megapixal over the D3000's 10mp. Plus the D5000 has live view and a screen that flips around and about. Not much difference but I find live view comes in handy now and again and so does the screen but most of the time I use neither. You can compare the specs here D3000 D5000
I personally don't think its worth getting a D5000 if you already have a D3000.
Im not sure what you mean by a lens suitable for landscape and portrait, I just turn the camera 90 degrees.