Can anyone tell me how the ProHD Cameras fare shooting SD? I'm talking low light shooting. How many stops better than shooting HDV? Is there any difference between 480 24p and 60i in low light conditions? and i know that we should probably be shooting HDV then downconverting, but is there a benefit of shooting SD over HDV in low light?

There is no difference in sensitivity between DV and HDV modes.

shooting 24P @ 1/48th vs 30p/60i @ 1/60th gets you about a 1/2 stop extra sensitivity, regardless of format ( SD / HD ). you can also shutter down more, or learn what black stretch and gamma are good for...