I have two identical computers (Intel Core 2 Duo, with ATI gpu). One 32-bit. One 64-bit. Some games only work 32bit, some only work 64bit. A few work in both... however I must say.... I had a lot more working on the 64 bit computer on 9926 which was in place upgraded all the way back to January. Then I formatted and clean installed 9926 and almost all the games that used to work no longer worked on 64bit. That has been my experience.
If you think it's working... you might not want to format and reinstall because you might have some artifact from an early build making it work.
I was working on a batch script that collected some diag info and a few other things... but a self referencing desktop icon on my desktop tricked me into deleting all the work I had on my desktop. Something I found to be interesting is that you can run DxDiag and get different results for 32bit and 64bit modes. On a 64bit system if you want to run 32bit dxdiag and compare the results with 64bit you will need to run it like this.
dxdiag.exe /t %DesktopReportFolder%\DiagKga-Dx64.txt
C:\Windows\SysWOW64\CMD.exe /c dxdiag.exe /t %DesktopReportFolder%\DiagKga-Dx32.txt
The first line will get you a 64bit DirectX Report on your desktop.
The Second line will give you a 32bit DirectX Report on your desktop.
That is the only way I have been able to get a 32bit Dx Report on a 64bit os. Perhaps there is another way that should have glared me in the face.
I also noticed on 64bit that I have no mantle drivers for my ati card where on the 32 bit os I have mantle drivers. but that does not explain the games that dont work on 32 bit with correct mantle.
Ghost REcon - 32bit yes, 64bit no
Duke Nukem Forever - 32bit and 64bit yes.
Archage - 64bit yes, 32bit no
Borderlands 2, both yes.
Counterstrike Go - 64 yes, 32 untested....
prior to clean install of the last known build i had them all working... then after... hit and miss.
everything needs to be tested more in both architectures... and then tested both ways again with a formatted-clean-install.