I just talked to a support staff of one domain name registrar, she said
I don’t think the domain name extension will have any effect on SEO'
If you just register a new .COM and .CO domains, and set up two identical websites, the crawl rate from spiders of Google, Yahoo, Bing or any other search engines should be the same.
Domain names are just indexes of a certain space with a certain IP (Name Server), the index itself has no values (to search engines); the only thing that search engines care is the site itself, the content, site structure, update rate and etc..
They will rank *exactly the same*, but keep in mind most people are not computer literate and either won't recognize a .nu (unless they live there) as a domain or will try going to www.nu.yoursite or www.yoursite.nu.com or something