Do read the link to Thom Hogan's advice on buying a tripod (link in jalagl's reply).
Then research the manufacturer's websites, as shops don't always have the full range for you to look at. The Manfrotto and Gitzo websites have great selection tools - I haven't looked at other manufacturer's websites.
Then buy the best you can afford, as you want it to last a long time. It may seem a lot upfront, but spread over even 5 years that a quality tripod will easily last for (if looked after) the $ are likely to be reasonable.