Running dxdiag in 64bit mode doesn't change anything in Windows 7. It just changes the mode dxdiag is runnig in. It still runs the same program, C:\Windows\System32\dxdiag.exe.
If you want to improve performance, add another 2GB or ram or upgrade your video card.
You can add a shortcut on your Desktop to run dxdiag in x64 mode by adding a /64bit parameter. There are other command line parameters as well.