To have HD calls with Lync (either 2010 or 2013) using RTV, you need a Quad Core CPU.
What Lync sends to the Meeting Server is a function of Lync, the camera and CPU capabilities (among other things). Window size is also a factor here.
What the Meeting Server sends to Lync is a function of what video we have to send to Lync and what Lync client says that it wants to receive. Lync sends us “Size Hints” during a call that tells us what it wants to receive: if it is telling us to only send 640x480, the Meeting Server must obey this.
With Lync 2013, you will only get 720p if the Lync window is full-screen. If opened as small window or even Maximized, it will only tell us to send 4x3 resolution @ 640x480. Therefore for HD ensure that you have clicked the Full Screen button.
Also, only in Lync 2013, screen resolution can make a big difference. In most cases with Lync 2010, we can get 720p on most Full Screen calls as long as the screen is set to a resolution of 1280x720 or greater. However, in Lync 2013 and using the same PC, it has been observed that screen resolutions of 1280x800 or higher are required to get the Lync client to request 720p in calls.