All inidcations I've seen (here at at AVS Forum) are that you will be dissatisfied with the performance of trying to stream HD files over wireless. HDTV needs ~20megabits/sec (plus the ethernet overhead data) so even a 54g connection under ideal conditions (direct line of sight) would be stressed.
If you've got HD (i.e. 720p or 1080i) content you intend to try watching via your Roku, stick with the wired connection. If you're only intending to do 480i (i.e. DVD) resolution, 54g wireless would probably be just fine.