Low end mountain bikes.....

[Trek420]

Bottom of the line GT, but I got Mavic Crossride wheels, much better and lighter

[DirtDiva]

xeney...
It really depends how much you want to upgrade and how soon. If you were going to do a bunch of things at once, you'd probably end up spending more money than if you just bought a whole new bike. The frame you've got is plenty decent enough though, and well worth keeping and changing out parts as you can afford them. Trek's right (of course ), lighter, stiffer wheels will make a huge difference to your ride. Disc brakes would be a good upgrade too (I'm pretty sure your frame/fork will be disc-ready, but you'll need to check that) if you're looking to improve your bike's performance. You can get not too expensive disc brakes, but the lighter ones are a bit more spendy. Stems and bars and seatpost are all good places to lose a little weight without breaking the bank (not that you can't break the bank on these items should you so choose... ).

[Trek420]

DirtDiva "Trek's right (of course ), lighter, stiffer wheels will make a huge difference to your ride."

heh, lighter/stiffer rider would be good too.

But seriously my LBS explained weight in the wheels is rotational weight, easier to get going. Stiffer wheels means less rolling resistance. If I was going to throw good money after bad at the bike that's the best thing to do. It's a cheap bike but it fits sooooo well and now...it's faster.