hey dave, build one.
the good thing is you can do it piece by piece & even start with your trek beater.
a mountainbike is as good as the sum of it's parts. the 2 important things are light weight & strong, along with reliable shifting while under torque. on a cheap bike everything will be very heavy & not necissarily strong either & shifting performance can suck... little by little you can just upgrade parts... then, at some point, when you're already on a good wheelset & have a good drivetrain, good shifters & a good fork & everything, then you could just start loking for your ultimate frameset, swap everything over & bam, fully built bad ass mountain bike!
I had a really cheap trek 820 steel frame bike & just for fun did a full xtr/sid race build on her with a set of kings with a light weight fast semi slick tire, great saddle & post, all great components top to bottom... everyone that saw trek 820 used to laugh at it, but I would kick everyones ass with it, it was great to beat out guys with cannondales, turners, a guy with a merlin & all there other big $$ bikes... they never fully realized that a bike is about way more that what it says on the frame...
honestly that steel framed trek 820 was an awesome riding frame... yeah maybe an extra pound or so cause it was chromolly, but steel is just so much more plush of a ride than either aluminum or carbon fiber or even titanium, the most comfortable hardtail I have ever riden & it was looked at as a joke, guys couldn't even belive a piece of crap bike like that could even live on the trails we used to ride at, much less outperform there megga $$$ snooty bikes
bike road so nice that I would really like to build anther steel hardtail or maybe even build that frame back up again, love the ride of steel but I'm a real weight weenie too so that kinda contradicts... honestly I think I really felt at zen on the trails on that bike more than even my titus at times...
if you did decide to do this the only real areas of concern would be to make sure you don't cut the steerer tube short so it'll fit the new frame eventually & also realize that you will probably need another new bottom bracket & maybe a different front derailer on the new frame so don't spend huge there, otherwise you can just start throwing great components on that beater without any real loss or waste in the end...