I've downloaded Isles and catacombs. Its my understanding that catacombs comes with trials of atlantis. Im currently downloading Darkness rising

What I want to know is do i have to download foundations and New frontiers or is that included in catacombs?

I've noticed theres a high res texture optional patch available should i download that or is it obsolete?

Both Foundations and New Frontiers are declared optional downloads - i.e. they are not included in other packages.

The high res patch you are talking about probably is called New Town Art Patch? This is only an optional upgrade for Catacombs. If you are installing Darkness Rising you already have improved graphics.

Both Foundations and New Frontiers are declared optional downloads - i.e. they are not included in other packages.

Both are contained in Catacombs
so you dont have to download them optionaly
also catacombs contains classic, isle and toa

Foundations and New frontirs are 2 Patches, you dont really need, to play, but its important to have em. They are NOT included in any other update.
Darkness Risind has only got graphic updates for Realm-Capitals, if im not mistaken, so, it wont be wrong, to download New Towns, too.

I've noticed theres 2 new frontiers patches. one for trials of atlantis and another for shrouded isles. I should get the trials of atlantis, right?

Yes, you might want to download the upgrade for the most current version you have installed.

I never downloaded nf. Actually just catacombs and i am quite convinded, that it is enough. I have nf, so it must be included in cata.

True. Catacombs comes with all previous expansions, including both Foundations and New Frontiers.

Here's a small list of the optional downloads needed for every client:

Super Classic: Needs Foundations and the SI version of New Frontiers. Since it's not possible updating this client anymore and due to all expansion areas being inaccessible I highly recommend you to get at least Trials of Atlantis.

Shrouded Isles: Needs Foundations and the SI version of New Frontiers. Lacks one character race per realm, therefore an update to Trials of Atlantis or higher is recommended.

Trials of Atlantis: Needs the ToA version of New Frontiers.

Catacombs: No optional downloads needed.

Darkness Rising: Don't know much about this product 'cause i haven't been around for a few months meanwhile, but from what i've read so far i believe a previously installed copy of Catacombs is needed in order to play Darkness Rising.