Depends on how the kernel was configured. Some distros, like RedHat, come with 'Enterprise' kernels that have highmem support compiled in for you.
Having an extremely high amount of memory can cause strange problems though, how much are you planning on using?
Also realize that even if you have 8G of total memory each process will only be able to address 4G total and I believe 2G of that is reserved for kernel use, giving each process an upper limit of 2G per process. There are patches to make the split 3/1 instead of 2/2 but I don't know how well it works, I know Win32 apps have to be designed to handle a 3/1 addressing split.
A normal, 'low-memory' kernel has a limit of ~800M.
A high-memory kernel can support 4G on normal 32-bit hardware
A high-memory kernel can support 64G on PAE enabled hardware.
edit: OK, so 16G seems to be the practical limit right now.