XML Editor

Would anyone recommend a free XML editor for Windows and/or Linux? I 
just want something that will help me write XML that conforms to the 
Gobo RelaxNG spec. Preferably it would have a window that concisely 
describes the Gobo doc format so that I can quick reference it while 
I am typing. So for example, while I'm typing a feature name for the 
first time I might think to myself, "I wonder if there's a feature 
name tag?" A quick glance would show that there is. The tool would 
have auto completion for the tags and would let me know if my doc 
doesn't conform. A spell checker would be nice too. WYSIWYG is not 
important because I can quickly use gexslt to transform it and 
refresh the web browser to view the HTML.

I tried oXygen already. I don't mind paying for good software (and it 
seems very good) but I think it is WAY more than I need right now. 
Plus I couldn't figure out a way to view both the RelaxNG spec and 
the target document.

If VI or EMACS is the answer, can someone help me configure it?

Thanks!
Brian