dude this has been discussed many times till date. wether it shows 3 or 4 GB, a 32 bit computer cannot address more than 4GB of memory space.

now, why 3GB then?
Since 32bit machines can address only 4GB of memory (32 bit address bus) and in pre-xp era ram used to be very less; The OS used to divide 4GB of space into two parts. 2GB for User space and 2GB for kernel space. but after the xp era, computers started having more ram; so they tweaked this logic so that 3GB can be addressed by the userspace and 1GB by kernel. This is known as PAE or physical address extention. thats why the systems used to show 3GB of memory since user space cannot see the kernel memory.

Now, in vista they addressed the problem of "incorrect calculation" of memory as 3GB.

On the other hand, linux does not have any concept of paging in kernel space. so linux doesnt reserve any memory for kernel. but if there is any app that allocates more than 2GB in kernel space, then you will see only 2GB available to userspace. (i.e., physical memory-kernel memory=user memory)

bottomline: if you have 4GB of memory, 1GB is exclusive to kernel and 3GB is for userspace.