Instead of upgrading your shock, which will cost you about $200 to $250 for a decent one; you should consider checking out ebay for a used frame & shock and swapping most of your components over. You've got a decent fork now and the frame is the heart of the bike. If you can get a deal on a new frame let's say $350 to $400 for a decent yet cheaper frame, you'll be better off that a new fork and rear shock on that Mongoose frame. Too expensive, probably not! There are sooooo many people (me included) that sell their bikes for less that 1/2 of what they paid for them and they're in great shape! You may even find a good bike with a crappy fork on it and put your new fork on the used bike!

L8R