How important is it to negotiate to buy the .com domain name to match the established .net website of the same name....
I don't always buy the corresponding names. If I can buy the .com. then I get it then look at acquiring the ,net and .info. If you are looking for a .com, and the com is taken, but the .net is available, I will sometimes take it, just for the type-in traffic.
It sounds like you are saying that you have an existing .net and the .com version might be available. I would think this issue is fairly important. New Surfers coming looking for your site are likely typing the .com first when they try to find you. If you can get the .com without major expense or risk to your current operation I would do so.
If you are starting a business, and have the funds to work with, I would buy every iteration of the domain name, including variations, using the example newtube.com, newtube.net, newtube.org, newtube.tv, newtubesucks.com. and so on.
Another thing I would advise. Buy your domains for a long period of time. For what it used to cost to buy a domain for 1 year, you can get the same domain for 7 years. When I can, I buy the domain for at least 3 years. Some search engines give you better ranking if they see your domain is registered for years, and not expiring in the next year.
It is also more professional to have a situation where you are not having to keep domains updated every few months. Anyone remember the embarassment when hotmail.com's registration lapsed? You can bet Microsoft will never make that mistake again.