A 128GB SSD is good enough for Windows 8, but it depends on what other programs you want to install.
I believe a clean install of Windows 8 takes around 18GB. That will increase when updates are installed. Virtual memory (the Page file) is typically equal to the amount of RAM that you have; if you have 8GB of RAM installed, then the Page file will be 8GB. Generally speaking, people say that if the PC has 16GB of RAM, then there is no need to have a Page file in Windows assuming the programs loaded do not use more than 8GB of RAM. If you want the laptop to be able to hibernate, then Hibernate.sys file will likely be around 5GB - 6GB. Disabling hibernation will free up that space. Then there are Windows restore points which I believe...