as far as a good budget crank and bb, id definatly suggest a raceface ride crank. it comes with their x-type external bb and is adjustable between 68-73mm. as far as cassette, as someone said you probably have a freewheel on a budget mongoose. either new wheel/hub, or just stick with the 7speed. i really wouldnt suggest an 8spd freewheel. i tried one before i switched to a freehub and was bending axels every week or so.