Are you talking about buying a domain name or registering one? There are different criteria for each. If buying one that is already registered it depends on your budget and what you are looking for. If you a registering a new one stick with a .com and get one with good type in traffic keywords that fit your business niche.
You might want to check out this useful article : http://www.naturalsearchblog.com/archives/2009/08/23/10-tips-for-buying-domain-names/
Welcome ICTclub, it is important to know to buy the domain name in my opinion is an effective, attractive, not easily forgotten and are global.