The size for the HTC EVO is 960*540 pixels.
The Admob banner is 320*50 dip (density independent pixels). This is equivalent to the number of pixels that would be used on an mdpi device.
Since the HTC EVO is a hdpi device the banner will get scaled up to 150% of its size so to approximately 480*75 pixels.
You should be thinking and designing in dpi not pixels.