You can take an mkv of any quality and convert it to an avi (or any other video file type for that matter) with ffmpegX. It’s an incredibly powerful bit of software - I use it to create all of the flash videos I post on my site.

Simply drag the downloaded file you can’t watch into it, and make the output something your computer can handle - such as an avi. Reduce the resolution from 1280×720 to something reasonable for your processor - I generally go for 592×336 since that’s the size I use for my posts. There are many other things you can tweak, such as audio and framerate, but I tend to leave these alone at this stage since that’s not going to kill the performance. Make sure to click the ’subtitles’ box on the filters menu if you are starting with an mkv, and you are all set.

Now let it crunch away for a bit. It might take some time depending on your processor speed, but you’ll end up with a file that will play easily on your older computer regardless of the player, be it Niceplayer, VLC, Mplayer, or what have you.

The quality will be reduced from the original file, but not too badly.

Maybe this is what you were asking for? It’s a bit clunky but works like a champ. Shoot me an email if you need help.

Kayle is right in offering the best solution - Perian + Niceplayer is by far the best solution for watching mkvs on a mac. For non-HD files it works like a champ.

The problem is that now everything is going to HD. I suspect your ‘aging mac’ is much like my own - PPC based. Unfortunately that cpu just can’t cut it anymore. Niceplayer and Perian are the solution, but you need a processor to back it up. Sorry - not the answer you were looking for, I’m sure.

I’m eagerly awaiting Kaiba as well. I want to see what the guy who did ‘Cat Soup’ can do with this one.

I’m in love with Kurenai. It’s early yet, but I think that might be the best one of this season. Give it a little time to make sense before you ditch it.

I’ve been dragooned into typesetting for Kurenai, haven’t seen the first episode yet though.