My Sony HDR-SR7 has four options for HD recording quality: XP (with a bit rate of 15 Mbps), HQ (9 Mbps), SP (default, 7 Mbps), or LP (5 Mbps). How do these options correspond to resolution (width and height in pixels)?

All the clips I've shot so far were done at default quality. My video editing software (Final Cut Express 4) says the frame size is 1440x1080, which is the max for this Sony model (although I understand that models coming out later this year will be 1920x1080).

So if the frame size is already maximized, what do you gain from recording at a higher bit rate? Clearly, you lose out on hard drive space at such higher qualities.

Thanks in advance.

while width and height remain the same... the bitrate determines how accurately that field of pixels will be recorded. The lower the bitrate, the more you'll notice macroblocking type artifacts. Basically, the more data/information per frame, the better it will be recorded/reproduced, less data/lower bitrate, the worse the result will look.