http://www.cultofmac.com/302102/11-awes ... -old-macs/http://www.engadget.com/2010/04/07/12-t ... -computer/http://www.makeuseof.com/tag/are-there- ... werpc-mac/Software-wise, I think it'll run up to Adobe Creative Suite CS4 (possibly CS5 but it's a grey area, check the specs carefully). You could run it as a media server but that does seem like a waste really, it will hardly break into a sweat doing that.
You might struggle with web browsers - there is a port of Firefox called TenFour Fox which could be your best bet. Flash Player support ended with version 10.1 - I've got the installer backed up somewhere - shout up if you need it. Even with v10.1 installed, some sites will still not play video - Facebook being one example. YouTube can be configured to use HTML 5 video instead of Flash, you have to run Safari 4 to run HTML 5 video I think.
Will have a think if anything else springs to mind - think I have some software installers I could send you if you want.