Windows would load faster and any programs on the SSD would get the boost, files on the hard drive will still run at hard drive speeds.
That said, personal files do not require the fast access times of an SSD. It is still a step up for loading your programs from the SSD.
You can move your desktop and all documents(pics/videos/downloads ect) to the hard drive to save space.
Only files on the SSD will get the SSD speed boost. so pick the programs that require speed and install them on the SSD. I highly recommend taking your desktop and files off the SSD since they desktop tends to fill with large files(or at least mine does).
No. Putting the OS on the SSD will mean you boot in about 2/3 the time. For example, have a box upstairs w/ a 120GB Vertex 3 Max IOPS and a Seagate Barracuda XT ..... the SSD boots in 15.6 seconds..... booting off the HD boots in 21.2 seconds.
Opening up a program or file on the HD will depend on the speed of the HD not the SSD