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And what's your question?

You can install Linux on macs, it's not a huge deal, but you should do some googling about it. It should handle pretty much any distro.

It is possible. There are plenty of videos on Youtube and info online. Ubuntu used to have a tutorial on how to do it as well.

Enough power? You do. I dare to say that all of the DE will run with that HW. Don't worry about it.

You don't need a special iso for macs. That was meant for the PowerPC machines. Since iMac 2009 is definitely an x86 based pc, you just need a regular iso. I recommend 32 bit, it being less memory-hungry. You should be just fine with using regular Ubuntu, I installed Xubuntu on 2007 plastic Macbook with 2 Gigs of memory and it works better than Snow Leopard.

There was a really good guide I saw on YouTube about how to make a bootable Linux install USB from the osx terminal. Maybe give that a google.

I've installed multiple distros on Core2Duo e7200 2.53GHz (OC = 3.17GHz) with GT210 nVidia and 4Gb of RAM, and it eats every distro, no problem (aside from distro specific bugs).

So hardware related, for sure there is enough power for even internet usage, HD (1440p+) playback etc.

Considering your main question, it should work with any Linux distribution, but is there any catch with MAC, I don't really know.

Here is a guide:

and the search term I used to find it:

ubuntu imac 2009 guide

The steps up until the installation there should apply to most of the mainstream distros.

Although those iMacs are worth a pretty penny so it may be worth selling on and cashing in and buying yourself a nice Intel Nuc system to play with or something along those lines.