The "64 bit" thing refers to memory addresses (actually 63 bit). Previously, 32bit memory addresses maxed out at 2^32 = 4,294,967,296 bytes (4GB). This means that Logic and all plugins could only "see" 4GB (except for EXS24, which was recently given its own 4GB of space as long as you had 5GB or more physical RAM). Even if an individual 3rd party plugin was coded for 64bit addressing, it was still treated as if it were 32bit.
Logic in 64bit mode can see 2^63 = 9,223,372,040,000,000,000 bytes (9EB aka ExaBytes). I found out it wasn't actually 64bit because I have 12EB in my Mac and Logic didn't see it all.
If you only have 4GB to begin with, the ability to address more than this won't be useful to you. In fact, there are several things that aren't 64bit compatible yet (Apple has listed them, such as Firewire Video playback, Euphonix control, etc.....).
Don't confuse the concept of memory addressing with audio engine. Both are measured in bits but have nothing to do with each other. Angles, temperature, and separation from Kevin Bacon are all measured in degrees, but they also have nothing to do with each other.