There are loads of 64-bit processors around, but what determines whether or not a system
is 64-bit capable is the operating system, which in the case of Windows nowadays means
Vista 64-bit. Most audio interfaces do now have 64-bit drivers to accompany it.
However, the majority of musicians are still perfectly happy with the 3-4GB of RAM that
you can get with 32-bit Windows, and only those who routinely load in loads of samples or
RAM-hungry softsynths find they need more. Personally I'm still happy with 2GB of RAM
Unlike your music requirements are really ambitious, I suspect buying a laptop running
Windows Vista 32-bit and up to 4GB of RAM will be fine.