i am using lots of engineering soft wares which i assume are 32 bit ...like various chip programmers, simulators ,...orcad design softwares ect .....i gonna buy new laappy ....with WIN-7 which comes in 64 bit OS .... so as have read revies it seems my old softs would have problem to work on win-7 64 bit....is that true? is yes ...i have been suggested to go for win-7 Pro-64 bit which have some XP mode by spending some 1000 bucks .....do it realy then supports the 32 bit apps ......or will it be just dont think much and dont do R&D and go with simple 32 bit OS ... ..also as i have read this 32bit/64bit is the OS thing, which also depend on what hardware is available, so does that meen 64bit OS would not work on my old lappy.....if yes ....then will my new 64 bit OS compatible lappy would be still backword compatible and let my OLD 32 bit OS to boot on it?
 I think a laptop with Win 7 32 bit would be the way for you to go. Since the XP mode on the Win 7 pro/Ultimate might not always work with the engg applications which you run.
 What are the specs of your old laptop? If it is more than 2-3 years old, then you might not be able to install Win 7 on that.
 Laptops usually come with their own recovery discs & it is not recommended to run OSes other than that on the disc; as the primary OS (though you can try dual boot). So if you get a Win 7 64 bit laptop, it might run 32 bit Win 7 only as a dual boot.
running CAD ware on a laptop is questionable at best, the graphics demand is tremendous, processor power in mobile chips is crippled, you should be looking at a real HIGH end lappy with lots of RAM, killer graphics and the best processor possible, attempting to run CAD apps in compatability mode may not work, but I have run ACAD in compatability mode on my desktop, but Catia and Solid Works wouldn't