I would either:
A)Scrap the mongoose now and get a nice bike.
or
B)Wait until you have more money and can afford a nicer bike.
Kona makes a line of bikes for larger riders, the Hoss and Hoss deelux I believe they are called. A member here (PWRDbyTRD) has one and he seems very pleased with it.
Edit: links
Hoss -->
http://www.konaworld.com/shopping_ca...1&parentid=182
Hoss deelux -->
http://www.konaworld.com/shopping_ca...2&parentid=182
BTW I know you asked for a FS bike but FS bikes don't start to get good until you get into the $1000+ range. I figured that since you kind of implied that a cheaper bike would be nice I would just suggest the hardtail.