You can use one that small but you will not be able to install anything but Windows on it so you get no benefit from having your programs and games installed on the SSD. Personally I think 64GB is even too small and you are better off waiting until you can afford 90-120GB.
I have a 120GB Vertex 2 and I have to be careful with uninstalling old games when I install new ones. You run out of space faster than you would think. I only have Windows 7 x64 Ultimate, Office 2007 and programs and about 8 to 10 games on the SSD. I have about 30GB free. Also factor in you can not totally fill up the SSD and you need to leave 30% of the space free for TRIM to work correctly.