I talked to a videographer for a well known Motocross magazine and he was saying he records at 24p with a shutter of 1/1000..his exact words .."I shoot at 24pn/24 frame rate at 1/1000 shutter. That freezes the dirt and gives more detail I think"
slowing down progressive footage whatever the shutter speed will give you choppy slowmo, you need to shoot interlaced (gives you more frames to work with) & using a shutter speed of 1/120 or higher will help give you smooth slowed down footage.
I suggest you search for slowmo with dvx on this site, there is loads of info if you look for it. for instance check out these threads for slightly different ways of achieving it depending on what software you have
shoot 24p for all the other footage and is 1/1000 is fine, but it depends how much light you have, I've tried filming with my shutter at 1/1000 and the footage came out too dark.