Frankly the 64 bit versions of XP and Vista are rife with driver issues, compatibility problems and more. Many well known hardware and software firms are not writing 64 bit drivers / versions for 64 bit machines.
Frankly it's more trouble than it's worth at the moment. Go for 32 bit for now. You will regret going down the 64 bit route I fear. I have a handful of clients who have, and nothing but grief has ensued.