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#1 The export format for an object is called DTS, and the form an animation takes (if not inbuilt) is DSQ. Most 3D modeling programs do seem to have exporters. Look in the artist section under documentation that is under the SUPPORT button in the header of every page on the GG site for more info.

Also "DTS exporter" and your "favoured modeling program" in google are usually good for some extra links and info.

As for recommending a modeling program ... it kinda depends on your budget. I use Blender, it's free, the interface is initially baffling. Whatever you choose you'll still have a fair bit of learning to do if you are going to get anything done.

GarageGames sells "Houdini" from the store. I've not used it, so I can't say what it's like.
www.garagegames.com/products/houdini

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#3 Constructor is really for BSP type buildings only. So it's all brush based blocks. A bit old school.

I'm not certain whether there is an exporter for Zbrush, you might have to do a bit of searching.

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#5 Xbrush is too high poly. You will want to create a low poly in another program and export the OBJ file to Zrush or create a lower level detail in Zbrush and export that to another app liek XSI,MAX or Maya.

#6 can you give me any idea of an object is jump continuosly with a constant speed.. I have done an object jumping but the problem is that its speed is constant but it move with very high speed.. can you give me any idea of this type of thing