I apologize for this being a bit of a long post.
I was in the same position as you about a year ago. looking to go one further with my MTB photography and looking at either the 3100/3200 or the D5100.
In the end settled on the 5100 because its low light capability makes it more suitable for use in wooded areas which, although not dark to a human eye, can be difficult for a camera.
Having since had the chance to have a play with a 3200 I was glad I splashed out the little extra on the 5100's better specs, the 3200 is a very beginner friendly camera.
As for lens choice I disagree with people shunning the 18-55mm kit lens. I think you are looking a top level commercial work or printing extremely large then you would need better but for getting started or mostly web and small print (A4) stuff the kit lens is more capable than my photography ability.
I would also highly recommend the 35mm Nikon DX lens. Good for very low light stuff and I use it a lot for portrait style stuff although difficult to get looking sharp at wide apertures.
The third lens I have is a 70-300mm sigma only a cheep one and I've only used it for kayaking where I cant get close to the subject. Its a little too long for the sort of MTB shots that I get.
Here are some photos from the D5100 using the various lenses. Click on them to view bigger.
D5100 : 18-55mm @18mm : F10 : 1/40s : ISO500
D5100 : 18-55mm @45mm : F8 : 1/400s : ISO100
D5100 : 35mm @35mm : F1.8 : 1/1000s : ISO1000
And less relevant to the MTB stuff but one taken with the Sigma
D5100 : 70mm - 300 @78mm : F11 : 1/1600s : ISO500
Hope this helps as opposed to making people fall asleep