If your TV doesn't support a specific resolution, expect to go low-res.
So if your HDTV can't accept 720p natively your games will be down converted to 480p
As it turns out, gamers who own older HD sets that feature only 480i, 480p, and 1080i resolution input capabilities will have to settle for the display quality being downsized as the game boots in its 480p mode rather than upscaling the image from its more desirable 720p mode to the TV's 1080i. We tested this development on older HDTV sets with games designed for 720p but not 1080i -- Resistance: Fall of Man, NHL 2K7, Tiger Woods PGA Tour 07, and Need for Speed Carbon. Sure enough, the system downshifted all four titles to 480p rather than moving up to 1080i.
So if your HDTV can't accept 720p natively your games will be down converted to 480p