Take a domain name with your business related keywords included that is help to ranking keywords another if u have already domain name then don't try to purchase new domain name becoz goole also look the age pf the domain name. if u want to expand your business in regional then take regional domain name.
Is there much difference if the domain have moore keywords than your main keywords. I think like if www.keyword.com is taken is the domain www.keyword-blog.com a good choice? I would think that the exact mach in keyword is best... But what if the keyword is "long tail" like the keywords discussion forum, is www.discussionforum.com better than www.discussion-forum.com? I would mayby think tha the last domain is better in that case?