Buying a tripod can be something you do once or it can be something that you do over and over again in some form or another. Start here. A good read:
Tripods and Ball Heads by Thom Hogan
You don't have to spend the kind of money Thom does unless you are using or planning on having the kind of gear he does.
A few tips for purchasing. These happen to be my opinion:
1. Plan for the future, not today. If you see your self with a 400mm f2.8 sometime in the future, buy for it, not what you have now.
2. You want a tripod and head that will support at least 2X's the weight of your gear, either what you have or what you estimate you will have in the future. This will give you the kind of stability you need for all kind of conditions.
3. Purchase a head with the gear in mind. Not just the weight of the gear, but the type of gear. Many head manufacturers like Manfrotto have proprietary QR plates/systems. If they meet your needs now and in the future you are good to go. If not, look to spending a bit more on a QR system that is Arca-Swiss compatible now instead of switching out sometime in the future. There is no way I would mount my 400mm f2.8 on a tripod with a Manfrotto RC2 plate on the bottom. I have no worries mounting in on my RRS BH-55 with the Arca-Swiss lens foot on that lens.
4. Look at your planned needs and buy the material that meets those needs. There are three types of material that most tripods are made from:
Aluminum = Cheapest of the three, medium weight to carry, good stability.
Carbon/Boron fiber= Most expensive of the three, lightest weight, better stability.
Hard Wood = Medium priced between aluminum and carbon/boron, heavy, best stability of the three.
5. Buy a tripod that is tall enough that you do not need to extend the center column for most situations. Extending the center column causes a loss in stability.
6. If possible, go to a photography shop that carries the model(s) you are interested in and try it out. Get a feel for it before you buy. Make sure you like the way the leg locks work, column work etc.
There are lots of good manufactures out there that make good products. If you choose wisely now you can end up with a tripod/head combination that will literally last you a lifetime.