Our TV signals are 50Hz (PAL). DVD movies are mostly 60Hz (NTSC).
On your second question, If your LCD TV is 720P (which is the case of the A53M) then
1) if you select 1080i in your DVD player, your TV will scale it to 720P
2) if you select 720P in your DVD player, it will pass through untouched (720P).
3) if you select 480i or 480P in your DVD player, your TV will scale it to 720P.
The rule is whatever compatible signal you input to your LCD TV it will always be scaled to the native resolution.
btw, the purpose of a upscale DVD player is to upscale normal DVD (which is 480P) to 720P/1080i. Although your TV can also scale 480P to 720P , stand alone upscale player normally do the job better. That's why people buy good upscalers e.g Sony 76H, instead of letting the TV do the upscale from a normal (Non upscale) 480i/P DVD player.
But if u compare sharp 100Hz and samsung 50Hz for Astro, then u will be able to see the different, the picture in Sharp is better compare to samsung ? do you have any idea?