Advantages:
- Slightly faster - probably expect around 10-15% speed gain
- Ability to address >4GB memory
- Can run both 32bit and 64bit apps

Disadvantages:
- Larger RAM/HDD footprint
- Older devices may not have 64bit drivers
- Quirks of WOW64

Not really that much of a difference.  IMO not really worth setting up a new OS for.
Your games should work, as well as most applications.  Some lower level apps may not work (ie RAMDisks, firewalls, etc).  Drivers, you'll have to look at them.  And you can use 32bit codecs fine on 64bit Windows.