I've been studiously ignoring this thread based initially on its title, and later on the number of posts (almost always a bad sign

), but now that I've glanced through it...
I'm with the camp that says the performance difference between x86 and x64 is not "night and day". That's pure hyperbole. If you don't currently have or need the extra RAM that x64 supports, there is virtually no point in going there right now.
I
did mainly because the "builder" copy of Win7 that shipped with my Quad processor upgrade did not include the x86 version, and partly because I thought the
promise of improved performance, no matter how modest, would be nice. I only have 4GB of RAM, and do not currently own any synths or create projects that need more, so that was not a consideration.
Through much playing around, and some deliberate testing, I have been able to discern exactly
zero performance improvement, and there have been a number of costs:
1. Loss of x86 DXis.
2. Loss of Rewire support.
3. The Bitbridge timing bug that adds an empty buffer of audio to some synth outputs (Dropzone to name one that is included with SONAR).
4. Other Bitbridge awkwardnesses and instability related to its running as a separate application process.
5. Buggy x64 drivers for my audio interface.
Some of this can be addressed by running x86 SONAR, but I prefer not to have to maintain two installs, and have been willing to put up with the compromises.
The Bitbridge problems were addressed by running most x86 plug-ins under Jbridge.
The x64 E-MU driver problems continue to plague me, and will necessitate a pricy interface upgrade and associated learning curve.
All things considered, I probably would not have made the move had I anticipated all the consequences, and if having my DAW work as well as possible were my only goal. I blame only myself for this choice, though the few datapoints I could find on how the E-MU would work under x64 were misleadingly positive.
But I have to say, also, that I like to stay somewhat current on technology (and need to for my work). And for that reason, I haven't been massively disappointed in the "upgrade". It's been a valuable learning experience.