I have a gary fisher tassajara, and I was thinking about upgrading some components on it. That lead me to the thought of going full out XTR(not going to happen), but theoretically if it did wouldn't my frame ( excluding other crap) essentially be the worse part of my bike? Would it be a waste to upgrade that far?
Are there actually signficant differences in hardtail frame quality?

It just seems that if you had a frame that you were comfortable on and comfortable with the weight of, that there wouldn't be much else reason to look for a new one. Am I way off here?

How old is it?

2005 I believe. It was bought in january.

The Tass is a nice frame. It's not PRO level, but then again, neither am I, and I'll assume neither are you.

I'm not a big fan of upgrading for the sake of upgrading. I only recommend doing it when a part breaks or wears out, or you have some significant performance gain.

The Tassa is what I would consider an entry level race bike. Meaning it's of the quality you can line up at the start of your local XC race and be competitive w/o the bike hindering you.

IOW, it's a good bike.

With that said, if you have money burning a hole in your wallet, I'd recommend upgrading the fork first. It's probably the weakest link. It's not a BAD fork, but more of a "middle-of-the-road" fork. You can find some top quality forks on closeout deals that will significantly improve your bike.

Other than that; Tires.

Tires can dramatically change the way your bike feels. It's also a great place to drop weight on your bike. Remember rotational weight is more signifcant than "actual" weight. Meaning the weight of the tires/rims will make more of a "FEEL" difference. Lighter weight tires/rims will make the bike FEEL faster.

I recently did an upgrade on a K2 Zed frame for that very reason and feel confident about it for two reasons. 1. My area is mostly flatland, the few river bluffs I ride probably won't stress a decent frame much! 2. My riding style is not too agressive and mostly XC riding, again probably not gonna stress a frame to it's full potential. You asked if there are significant differences in hardtail frame quality, Yes.
Will I ever notice them in my area, with my riding style? Probably not. If Iwas in British Columbia, or the American rockies, or Kula in Hawaii I would be more picky about what frame I built. In the end your riding style, skills, area, terrain, and what you plan to do with this bike should help answer most of your concerns

alright, so what I have gathered then, is that there can be significant differences in frame qualities, but since I ain't exactly race material I won't notice a difference. It would also be a waste to upgrade the drivetrain components a bike of this level that far because for that much money I might as well upgrade the whole bike. And in conclusion the bike I have currently will not be holding me back anytime soon, as long as I ride like it was made to be ridden, but at this level a good first component to upgrade would be a fork or tires.

Thanks for the info.

One other point: upgrade as you go along (per a2psyklnut suggestion), but don't be afraid to UPGRADE when you do since you could carry it over to the next frame you get (forks may not be compatible, so keep that in mind). I did this with my '92 Trek. I put wheels on it and shifters that were more than double the value of the frame, but they all went on my Attitude frame when I got it!

Right on then, yeah, I threw an xt derailler a friend gave me on there. He's a nice guy. So I am not really afraid to upgrade just wondering how futile it would be without a better frame. Thanks for the info, and that is a good point about being able to carry the components over.