Just saw this deal at xbox.com and thought I'd post it. M$ is offering used 20gig 360 HD's for $29.99 (plus shipping, I presume).
http://www.xboxstorageupgrade.com/hh/web/home/hhauthentication.jsp;jsessionid=8D9CF385CC0C6AC7972F03B242617363?ssLocale=en_US
If you've got an Arcade model, why...