Global Trademark registration is a very expensive (6K - 8K GBP per Mark), time consuming process, that is very murky when it comes to domains.
The only real protection it affords you is the ability to take other people to court (which you probably can't afford to do at the moment). You can send cease and desist letters, but many people would simply call your bluff.
If you really want to protect your online brand, you're better off buying all the TLD's you can afford (much cheaper) and hanging on to them until you're a larger company and can afford decent lawyers.
It's worth remembering that many countries, the US included, do recognise common law trademarks - so even if you haven't registered you would still have some avenues of legal recourse if there was really a problem.
Wiki has a really good article on this subject.
I'm not a lawyer in any country, this isn't legal advice, it's opinion.