daniel unfortunately a lot of people are giving you guesses on how to resolve your problem.
As someone who has been dealing with the same issue on a similar system for the last 2 months, i will tell you that it is neither your system OR vista 64 that are causing the issue.
It is now accepted by Adobe that long form projects such as yours bring premiere to its knees, no amount of ram or processing power will fix it because the software in itself is broken.
CS4 WILL work quite happily assuming you cutdown the project into smaller pieces to work on, this is the easiest way to operate until they patch the software, something that Adobe have said is in the works.
Trust me, its NOT you. The software at this stage is just crap.
~Peter Berthet
Sydney, Australia