If the site is good and backlinks are good it should not matter which type the domain is.

Meh. I'd stick to a dotcom myself personally, but then again, that's just my personal bias. I think .anything (besides com and net) typically look n00bish. But then again, if the price is right. The best domains are free domains.

Let me warn you Jaouadk. I tried the dot tk route when I first started. I had a great product and great site going and guess what happened. dot tk said I was violating some ToS (which I wasn't) and they took my domain. They then parked it with Ads related to the content I had on it. Spend the money and get your own hosting which you can get for .01 at hostgator for the first month and at the very least get a .info which is $1.99 at Godaddy or get a .dot from 1and1 for .99 but watch your statements close with 1and1.