You can also use Micro Niche Finder, is a great piece of software and I'm sure that it will help you a lot.
use the keyword tool (as above) and search for your primary keyword. you will also get relevant keyword with monthly searches which will give you an idea. find the keywords first and then do a little research to measure the level of competition
What about the possible ranking on Google ? How we can judge which keyword will be on top easily or not ? Please share some views on that as well.