Hi all.. this is my first post in this forum, so I'm a bit nervous
I've been dabbling with XML before, and the only decent alternative I found to Altova's XMLSpy is Oxygen (http://www.oxygenxml.com/), available on OSX too.
Also, I remember there's a feature in the LibXML2 libraries included in PHP called pull parsing, that could be useful to handle big files. I haven't the need to work with it, but seems useful (http://www.ibm.com/developerworks/xml/library/x-pullparsingphp/index.html)
Hope it helps.