lets say i want to register a domain name... Is it legal to get one like www.(something)myspace.com/www.(something)google.com/www.(something)ebay.com/www.(something)megaupload.com... and run a service similar to theirs? But different! or will that get me in trouble?
In answer to your question and myspace example, myspace.com has already filed a number of legal actions against owners of similar and misspelled versions of the name, and I am sure will be filing even more cases.
MySpace will sue if they so much as sniff that you might maybe possibly be thinking about doing something that remotely infringes on their trademark.
Whether it's right or wrong really isn't even relevant unless you have the monetary resources to defend yourself.
If you want to have a service similar to theirs, you have a decent shot at going about your busniess unbothered... there are other sites that are similar to MySpace already. Using a domain name with "my" or "space" in it (and certainly with both my AND space) is just asking for a lawsuit.