The general opinion seems to be that C5 x64 is simply still not a reliable solution, but it has gotten better with version 5.5.2 and there are some workarounds.
Remember that you can have both the 32-bit and 64-bit versions of Cubase 5 installed on the same machine.
You can also keep your 32-bit and 64-bit plugins seperated in different folders. Have a look here for some suggestions: