Receptional_Andy - 3:55 pm on Mar 9, 2009 (gmt 0)

It's all relative, I'm afraid. The key factors are how "serious" the competition is, and how effective you believe your search engine promotion will be. Strictly speaking there are no unobtainable keywords, although you will clearly find it a cumbersome tasks to try to get into the top three for large brand names, and tricky also for highly competitive markets.

There's no way to choose a number and say it is too high - it would be entirely arbitrary. I would recommend you review the sites you would be competing against (their site itself, and their incoming links) and decide if you would be happy to take them on.