I have a new XD and find it clunky. It finally connected at 'N', but at a speed of only 65M, so it doesn't stream smooth over the network with Plex when the movie is a large resolution and all of the other streams available (when they work, EI fox doesn't) it spends forever buffering.
Depending on the file that you are transcoding and the processing power of your work station. Even a wireless-G router stuck at 54Mbps would do. Heck, it doesn't even need 20Mbps.
As for the Plex issue, it all boils down to what you feed and how it is being fed to your Roku box.
I can stream 20Gb video files from my i5-2500k workstation to my Roku and it works smoothly.
Try to add more RAM to your PC?