There are only 2 reasons to get the 32-bit version
1) you have a 32-bit processor
2) you have legacy software/hardware that doesn't work properly on Windows 64-bit platforms
Both are incredibly rare for a casual user. A casual user would be someone who uses Windows Office, Photoshop (or Gimp) watches DVDs, plays music, uses the internet, and plays games, but doesn't hold onto 20 year old software.
Point 2 may be a factor if you are a programmer or designer of some kind, as you aren't using mainstream software that may not have been updated yet. Point 2 also is a big factor for large corporations that live on legacy software (ever go to the hospital and wonder why they are still running DOS programs 
)
EDIT: forgot legacy hardware, obviously check your the companies drivers page for 64-bit versions. Probably not a problem though unless the hardware is really old.
