Have no fear this holiday season. If you buy a new Vista Compatible PC between Oct 26th, 2006 and March 31, 2006 you will receive a free upgrade to Windows Vista. All the information is available here at the Microsoft Vista site
Each Tier 1 manufacturer (Dell, IBM, HP, ACER) will have their own programs as well as system builders (like Sysguy).
Also being offered are low cost cross grades, for example XP Home to Vista Ultimate or XP Pro to Vista Ultimate.
Basically if you get a PC with XP Home you will get Vista Home Premium and if you are running XP Pro you will get Vista Business.
For more info visit the Vista info site for more info on system requirements and for tier one contacts.