Most programs fits well on <4GB address space but needs to use new features just available on x64 architecture.
Are there compilers/platforms where I can use x64 regi
I'm afraid that if you are concerned about the size of pointers you might have bigger problems to deal with. If the number of pointers is going to be in the millions or billions, you will probably run into limitations within the Windows OS before you actually run out of physical or virtual memory.
Mark Russinovich has written a great article relating to this, named Pushing the Limits of Windows: Virtual Memory.