64bit processing is seen with a 64bit edition of Windows like 2005 release of XP Pro64 being the predecessor to Vista's 64bit editions. AMD claims that simply running a 64bit edition of Windows reportedly sees a 10% speed increase as it utilizes the 64bit side of the 32/64 cpus available for the last 5yrs.
64bit editions of Windows see backward capability for 32bit games and apps similar to the way WIN 98 saw support for a select number of older 16bit games and apps run on Dos, 3.1, and 95 in years past. XP will install onto a Fat16/32 primary partition as an NTFS type.
For many things Vista will run things very close to what was seen in XP. But there will always be some degree of variations seen which switching from one version to another. One complaint often heard in years past and still continues is the need to upgrade many softwares for each new version of Windows.