Well you can purchase an upgrade to Windows Pro or Ultimate which supports up to 192GB;
http://msdn.microsoft.com/en-us/library/windows/desktop/aa366778(v=vs.85).aspx#physical_memory_limits_windows_7 or some equivalent version of Windows 8.
As far as better 4x4GB or 2x8GB: 8GB offers the potential of 32GB but 8GB/stick non-ECC RAM is significantly more problematic for stability. Therefore if looking at 8GB/stick kits then DDR3-1600 CAS 9 or 10 @ 1.50v. The 'faster' kits are inherently more problematic, AutoCAD similar to most Rendering applications achieves, CPU-to-CPU, it acceleration more so from abundance of RAM than Frequency and CAS...i.e. more is better.