http://bythom.com/support.htm
http://www.luminous-landscape.com/essays/tripods_&_heads.shtml
Your tripod is about as important as your lens in terms of getting sharp pictures. You cannot hope to achieve the resolution of your lens, in most cases, without a tripod or some means of supporting the camera especially if using telephoto lenses.
There is also a study that was done by Leica (and not available on the web due to copyright) that shows that heavier tripods (no surprise there) do the best with vibration coming from the ground up and CF tripods do better with torsional forces (like wind etc...).
You can typically make a tripod heavier by hanging a mass like your camera bag either around the legs or from the bottom of the center column (hook). Stiffness in the legs is the prime contributor to resistance to torsional forces and is much more difficult to achieve since in many cases, increasing stiffness also can increase the vibration coupling from the ground. This is where the science and engineering come in.
If you read the markins article, then you realize that we are talking about vibrations here that are to small for you to detect or feel (on the order of 5-50um or so). Because of that, you really cannot 'kick the tires' of a tripod and tell how it performs unless you take one out and make some high detail shots and see how it performs or to do a test using lasers and long, long lenses as Leica did. It is not possible to tell by whacking a tripod and seeing how 'stable' it is to know how it will do in reducing and isolating the camera from small microscopic, sharpness destroying vibrations. A whack and the vibrations we are talking about are many orders of magnitude different.
Because of that, and because engineering to achieve these results are expensive the better tripods are also expensive. My rule of thumb is expect to pay for one with a ballhead what one would pay for one of your better lenses. Seems to hold relatively true.
You won't want to hear my advice on this - but I think you would have a hard time getting a decent tripod and ballhead for $150. My advice would be to treat it as a major purchase like a lens and save for it in the same way and then get the good stuff. In that way, you will avoid the expensive iteration to get there eventually but with having bought several cheaper and less capable tripods in the meantime.
I guess, in my opinion, it all gets down to how serious about photography you are and how long your view is. If you are serious and the view is long, then the dollars per year is much less since the better stuff (manfrotto top end and Gitzo) will last your natural life time. If you are less serious, then perhaps it isn't as important (or more short term) and getting the cheaper stuff to last a bit with less result is ok.
I've basically lived the Tom Hogan article and I have 3 increasingly expensive tripods sitting in the closet as a result. I'd have saved a lot of money, and had better results in many of my shots, if I had done it the right way in the first place.
Anyhow, my $0.02. YMMV.
J.