If you are buying a new PC, and have the OS bundled, sure get Vista (x64) with it.
If you are buying Vista separately (either oem, upgrade or retail) ... forget it. It's XP SP2 with eye-candy & DRM.
Edit I've read a few places that if you have WinXP Business, you can legally install & use WinXP x64 Business./edit
At worst, you can borrow someone elses copy of Vista x64, or download a copy, install & use it for 30 days.
You can purchase a key from Microsoft via the activation wizard, if you consider it worthwhile (atm I don't).