Would it work by adding only the tuner card?

http://www.walmart.com/catalog/product. ... long_descr

It seems to support Linux, however without knowing the details of the machine, I think it would be tough to answer this without more info.

Has anyone cracked the case on one of these?

Googling for "Linare PC" turned up this review of their Duron based $199 model:
http://www.osnews.com/story.php?news_id=6307

Also the company's own online store.
http://linare.zoovy.com/

I think you'd probably be better off spending a little more to get something that you know would work. I paid $500 for my box off various online retailers last February (mostly newegg.com). It'd probably be less by now.

I like the idea of a $200 open source PC though. The kinda thing you could give your mother-in law so she can get e-mails and pictures of the kids...