Heewa wants to dual boot into Windows on the Mac. Leo recommends doing it with BootCamp. BootCamp will automatically partition it for Windows and Mac and he can decide how much of a partition to devote to the Windows side. He can share data between the two, so that's good as well. BootCamp will also create drivers for Windows. He should have a minimum of 20GB on that partition for Windows, since he'll need room for the Windows install and programs he'll install there too.
Heewa also wants to know if 4GB RAM is enough or should he double it? Leo says that if he's running Windows in virtualization, he'll need more RAM. However if he's going with BootCamp and choosing which OS to load, 4GB will be fine.