Eh, when I was talking about 640:351 and 68864:37791 I meant frame aspect ratio.
Oh, god, I find this very complex.
Well 720x576 is the widescreen standard for PAL SD video (read: DVD). This is because it's an anamorphic format. If fact, the pixel aspect ratio is 1,21 and not square pixel as you may think.
If you want 16/9 with square pixel, it's 1024x576. That's if you have CS3, because in CS4 the Pixel Aspect Ratio (PAR) for SD (PAL & NTSC) have been corrected.
You will find more information about PAR here: http://help.adobe.com/en_US/AfterEffects/9.0/WS3878526689cb91655866c1103906c6dea-7f3aa.htm l
BTW if you want to work in HD, the resolutions are:
1280x720 for 720p
1920x1080 for 1080p (Square Pixel, or 1440x1080 for HDV with 1,33 PAR)
16:9 is a rough approximation.
The figure you see is a super accurate way of expressing the end frame aspect for PAL D1/DV Widescreen.
If you divide both 640 and 351 (ie, the terms in 640:351) by 40, you get 16:8.775. Seems pretty close