It depends on your current setup and your intentions.
64 GB is slightly on the small side for a boot drive with your applications but its doable.
If you have an Intel z68 or z77 motherboard they support something called Intel SRT (Smart Response Technology) which can use a max of 64GB SSD. If you have a mobo that supports SRT, I suggest you use it. Its easy to setup without reinstalling windows or migrating everything basically the SSD becomes a dedicated cache for the HDD and as far as your system is concerned it doesn't even know the SSD is there but the performance of your HDD will be drastically improved over time. Anyway this is only possible/useful if you have a mobo that supports it.
If you want to use your SSD as a boot disk for your OS and apps there are two options:
1) Perform a clean install of Windows and required Apps to your SSD and use the HDD as a secondary data storage device.
2) Migrate your existing Windows installation and apps to your SSD and once you test your new SSD as a boot drive, use the HDD as a secondary data storage device.
In order to accomplish 2 you must meet two requirements and obtain one piece of software: You must have had AHCI mode enabled in the BIOS when you initially installed Windows on the HDD. If not - you are most likely out of luck and need to do a clean install anyway. You also must be able to fit the drive you want to migrate onto the 64GB SSD. This might require partitioning the HDD and moving data to a second partition whilst cleaning up the boot partition by uninstalling unnecessary applications etc eventually isolating a boot partition that you can migrate to the 64GB SSD.
In order to perform a migration you will need a software that can do it. intel SSD's come with this software free but you can also purchase it.
http://www.acronis.com/homecomputing/products/trueimage/
If you can I highly reccomend using SRT, if SRT is not available I reccomend a clean install on the SSD but if you absoultey cannot do a clean install as a last resort you can try the data migration.
A 64GB SSD is great for SRT but if you don't have SRT or what to use the drive as a boot drive 120GB is much more convenient.