The main advantages of 64-bit over 32-bit are the addressable RAM space and the speed of execution of most applications.
The first only applies if you already have more than 3 GB RAM, the second only for native 64-bit apps.
The problems are that device drivers have to be 64-bit, as do certain types of applications, such as AV.
There's a useful 32/64 comparison on the MS site here:
32-bit and 64-bit Windows: frequently asked questions
It refers to Vista but would be equally applicable to Win7.
SD