I think 64GB is plenty. Even if the "usable" space is theoretically 55GB.
I currently use 44GB on my MacBook Air but that includes by entire 16GB iTunes library with several TV shows I purchased. It Also includes my 10GB iPhoto library, of which 2/3 are photos I imported for backup purposes. If I were to cut those things down I could probably fit in under 32GB (you can get a 32GB SSD 3rd party for ~$500).
I just checked my iMac, which is my workhorse machine I use for downloading anything I ever need to download as well as do all my serious work on, and it only has 55GB of used space.
Keep in mind that both computers have copies of Adobe CS3 suite installed on them, iWork & MS Office 2008 Home & Student, full iLife '08 suite, and on the iMac I also have VMWare (though Windows rests in another partition).
I store all my non-iTunes purchased movies and TV shows on an external hard drive, which does have 100-150GB of them, but it isn't always connected to my computer (only when I want to watch something). I never did understand those people who buy a ton of storage for their primary boot drive to store things they will view maybe a few times if even that.
I think if you only keep the stuff you immediately use on your computer and even some files you MAY need at some point like your music library and such, you should still have PLENTY of space left over.
The ONLY exception I can think of is if you do a lot of serious video editing since those files tend to fill up space quick. But if you are not a professional video editor then the 64GB SSD should be MORE than enough.