Read this:
http://www.bythom.com/support.htmIt should answer all your questions, including some you haven't thought of yet.
Basically you need a stable platform to sit your camera on that can be a handy fence post or a $5000 tripod.
Where you stop in the middle is up to you.
Personally I went down Thom's $1700 route.
I have two Velbons, a few no names and a $600 Gitzo with a $500 RRS head on it.
Guess which one I actually use for my camera!
Things to look at:
Is portability an issue
- how heavy is it? generally, the lighter it is the less stable. Good design and choice of materials can alter this. But Carbon fibre is more expensive than Aluminium
- How compact is it folded. If it needs to go into a backpack or suitcase a lot, a compact design is good. Otherwise more compact = less stable and more expensive.
What height is it in use. (don't forget the camera height in this) DO you need to stretch or stoop to use the camera? YOu can always lower a tall one, but you are paying for that extra height.
How low will it go. If you have a need for low level shots (e.g. macro) you need to get your camera close to the ground
Locking mechanism - do the legs slip? Is it likely they will in a year?
The most important question: how stable is it in use? Put your camera on it with your biggest lens, or borrow your wish lens from the camera store. does it droop or wobble?
Remember, a good tripod will outlast any number of cameras. And you get what you pay for.
EDIT: and don't trust the published figure for weight handling. The manufacturers are trying to sell you their tripod. Some companies are known to be optimistic. Try the tripods out yourself.