64 GB should be more than enough, really, but your space requirements may vary based on your usage patterns. If you like to have several big games installed, or use several operating systems (either virtualized or not), I hope your wallet is as fat as the SSD you may need in such case. Otherwise, get a big mechanical drive for secondary storage as f0dder suggests (or a NAS), and move your personal files there. You can even set up folder junctions pointing to the secondary drive if you run out of space (hi, humongous "steamapps" folder).
How come moving "My Documents" to another drive is messy for you, Deo?
I install all my games on a different HDD than my OS anyway, but regular programs, User directory, and OS just take up too much space. I've got an 80GB HDD now for my OS and I've already had to move things around to get space on it. And that's even after I recently lost the majority of my MP3 collection due to a few seconds of absentmindedness backing things up (or not) in preparation for a format.
I had attempted to keep things clean by making a small partition just for the OS, about 20GB. Then leaving the remaining of an 80GB HDD for Program Files. I moved My Documents to another HDD and the games on yet another HDD. I soon learned that didn't work so well because there are things that get installed that don't allow you to customize where to install them. And there are lots of things that get installed or placed in the User directory, including but not limited to temp files, Google Chrome, application data, etc.
As I said, I tried moving Program Files to another partition in Windows 7 and most applications would properly install to the other partition, but they'd still create shortcuts and other things pointing to C:\Program Files\ which caused a lot of problems.
I tried moving my User directory to another hard drive but that broke some applications and settings that were trying to find it in C:.
I've put my "My Documents" (including music and videos, etc.) on other HDDs and that works well enough, except Windows applies certain permissions (read: restrictions) to contents of My Documents, My Music, etc. and those permissions don't get applied to the directories on other hard drives.
It hasn't ever been anything major, but lots of little tiny (and frequent) annoyances that finally made me give in and leave everything on the same HDD.