64-bits = 64 fingers :)
What are bits?
|it's right here!
|watch out when counting by 4s
Now your computer, 20 years ago had only 8 bits; nowadays typical your consumer grade computer can have 32 (say older laptop) or 64 bits to work with. This means that your computer is able to reference (theoretically) 2^64 or 18,446,744,073,709,551,616 locations.
Work PC w/ Win7 - right-click My Computer, select Properties.
For this to work, everything your computer does, has to be able to reference 64-bit locations. Assuming your operating system is currently installed and running in 64-bits, you are good to go! Windows / Mac OS has found the drivers to necessary. (The article linked above has how to check your machine. )
If your computer has more than 4GB of memory, you will probably experience a performance increase running the 64-bit client. I wouldn't suggest trying this right before a raid, as a number of people have reported issues with various device driver conflicts.