If memory serves me right, even on 64bit, the standard malloc won't allow you to grab a chunk larger than about 2G. You can still have as many chunks as you like, but you can't have a single chunk larger than that. Of course, you can always use a different memory manager.
Depends on your platform, libc version, compiler, etc. Anything recent should default to just passing through to mmap for allocations that are a small multiple of page size or larger, and on 64bit systems you can happily mmap huge regions.
