To expand on what ignatzatsonic
said already ... everything you run has to be read from somewhere and initially loaded into memory. IF you WERE to segregate your components so the OS ones were on the SDD and the application ones were on the HDD, you would probably see little, if any, improvement in speed once the OS got loaded. Why? Because every time you launched an app, the OS would have to read it from the HDD -- which would be no faster than it is right now.
As to space needed, everyone's experience is likely to be different. I'm running 32-bit Windows 7 Pro, have only about a dozen programs installed and have a "data" partition for all my large files -- and Windows 7 takes up a little over 40GB of my 60GB partition on my SSD.
So, if your SSD is 64GB, you should have plenty of space to install and run Windows 7 from it.