How important is it to grab all available domain TLDs for somewhat popular site? (Alexa Rank: ~20k)
Getting ALL is a hard full-time job usually only done by companies with more money than sense.
I'd suggest you get all the ones for countries you're currently active in or likely to work in in the next 2 years. If there are any that you just have a 'hunch' about then go for them too. If there are any industry specific ones you way (i.e. .tv) then go for them obviously then also go for the other major ones like .net/org etc.
Other than that I'd leave the rest.
I disagree, this is somewhat imporant. the last thing you want is someone to own Google.org and use it as a site to write negative reviews about your site (google.com) get the main tld's for a prominent site. .org .net .com . If you can afford it and have a worldwide audience look at top levels for important countries such as .ca (candada) .co.uk etc.
Agreed with Chopper. Get the global ones and the active country ones. There might be the odd exception, like .co (Colombia) .tv (Tuvalu, tiny island sinking into the sea. The TLD is its chief export, I believe), but even if you do get them all, you're then looking at all of the variations (hyphens, common typos, suffixes). How much time and money have you got?