More bits means that data can be processed in larger chunks which also means more accurately.
More bits means our system can point to or address a larger number of locations in physical memory.
You should buy 64 bit now. They also support more than 4GB Ram since 32 bit has a limit of 4GB.
This is a pretty good guide as well: http://www.techsupportalert.com/content/32-bit-and-64-bit-explained.htm