I'm sure I'll get pelted with broken derailleurs and beaten senseless with an old Rock Shox Quadra 21R for saying it, but if you are a BMXer you might want to check out the Redline Monocog. It's a steel framed single speed mountain bike with no suspension at all. Kind of like a big BMX bike. A mountain bike that costs under $500 is not going to work very well at all anyway, so why waste money on gears that don't shift and a suspension fork that just bounces up and down like a slinky? Most single speed mountain bikers get there via the long way, but you can short circuit the whole thing and go straight to one gear nirvana.