Ask HN: Trademarking your Startup

I am in the final throws of launching my startup. A name has been chosen (which I have grown quite attached to), and now its time to address trademarking. Its a bootstrapped startup, using what time and money I can save from my day job, and $18k+ with single-class global registration is so far out of my league I was wondering:

1) Is a trademark important in this day and age? I already hold the domain - isn't that enough?

2) If i DONT have the trademark, could someone else register it and stop me trading under that name?

3) As a generic web application, what classes and communities should I be targeting to get good coverage for the least expense?

4) What sort of protection do other HN startups tend to go for when launching their products?

American, also not a lawyer.

1. Having the domain is a good first step. As you can verify the registration date, you can claim copyright. (not trademark) This might be enough protection.

2. Sortof, but at a cost so high, I can't imagine why they'd do it in the first place. You'd also win any legal fights in the end, but at great cost (Lawyers ain't cheap).

Most will simply steal the idea and re-brand, not try and usurp your fledgling market. (see Facebook Poke for an example of destroying the little guy)

Your primary concern should be if someone else has already registered a similar name, and _they_ have priority over you.

4. Start with a Poor mans copyright, by mailing a certified letter to yourself with the outline of the idea, domain, pitch, etc. As it's date-stamped by the government, you can have it opened in court to prove you came up with the idea first. Used by authors and screenwriters seeking claim to the original ideas they generate, so no one can copy the idea without compensation.