1 GB of Virtual address space (and not 1 GB of physical address space) is locked to the kernel.
@praka: Who should check his facts? Me? I've had enough of your obnoxiousness. Unlike you, I've programmed for the Linux kernel (and Windows kernel too, for that matter).
The Linux kernel has a 3GB/1GB virtual address split (by default) on x86 procs. Physical memory support beyond 4GB is possible in Linux by using PAE (on x86 that is). You can do that in Windows too - they just don't support it for non server systems since too many drivers are flaky on client systems.
Oh, by the way Praka, a little factoid for you: the Linux kernel does not allow kernel mode code to be paged out. It cannot handle that situation. The Windows kernel does allow a lot of kernel mode code to be paged out if necessary. How is that memory management for you?
I've said it a hundred times before - there are lots of things each of these kernels has right and a lot of things they don't. But stop your foolishness and discuss the issue rather than act as a fanboi who parrots stuff without knowing what the hell he is saying.