Dual Booting - Vista 64 - XP: APPS and Drivers

Unable to find information to clearly explain how applications and drivers are handled in dual booting of Vista and XP. Building new machine, CPU - E8400, 2 x 2 g RAM, GT800, 3 HDDs (150 Raptor, 2 @ 320 WD Caviar). Have read many concerns about Vista 64 regarding apps and drivers. Thinking best to partition Raptor for Vista and XP and dual boot. Basic questions include:

- Vista64 First Install?
- OS and Apps in each separate partition (ie Vista64 with OS & PGMs partition and XP with OS & PGMs partition)?
- If Application can be used with Vista64 or XP, do you install once in each OS Partition? Meaning, will I find myself loading apps and drivers twice?
- Is a logical thought when dual booting to consider each partition as individual computer?

Thank you!