If there is a company that is registered under .com, .net but i really love the domain and .biz is not registered under them, do you think that i still able to have any client after i register under .biz?
Yep, Matt said what I was going to. If you cannot get the domain, I'm afraid you're just going to have to pick a new name.
At this point, .biz is not quite as popular as .com. I wouldn't go as far as saying that it's unprofessional, but I think it's a definite advantage to hold the .com just because that seems to be set in many people's heads that domain.com - and not .net, .org, .biz etc - is the place to go. I'd be a bit afraid of losing a few potential customers because they though .com was me.
I agree with CrazyTech completely. Don't think it's unprofessional, just not quite a widely adopted extension for businesses yet. Tropitan's new marketing campaign is pushing their tropitan.biz domain, so it's definitely not that bad. But, like Crazy said, if you don't have the .com, be prepared to lose a couple customers in the confusion.