how to access low-competition?

This is my understanding of accessing a low-competition niche or keyword short of using Market Samuri: look at the top 10 in Google, and chose it if there are at least a couple PR 0 or N/As.

Are there other ways for this, or this this the predominant method?

But I'm also unclear about the method itself - should the low PR rule apply for the main niche keyword or any keyword that you will target (make a domain name from and compare against also having high search frequencies)?

In other words, if I'm looking to promote products in the cat sweater niche and:
"Cat sweater" has all PR > 0
"best cat sweater" has a couple PR 0
"best cat sweaters" has a couple PR 0

- Does this mean this is a over-populated market because the main keyword doesn't match the criteria?
- ...Or can we choose "best cat sweaters" as long as we make our domain that name and make sure searches are high enough?
- In general, is plural or singular better... Wait, I actually just figured the one w/ the greatest monthly searches is best all thing being equal - right?
- What if "cat sweaters," "best cat sweaters, even "top cat sweaters" and "cheapest cat sweaters" all had top 10 results PR>0... does this mean I should avoid this niche, or just find a keyword within the niche with buying intent that meets the PR criteria?
- Is the PR criteria a good one to begin w/?

Thanks for helping out. This is my first time doing this and I want to make sure I'm - generally, at least - doing this right!