It seems like a good thing to be going to 64 bit since the servers will run faster and be able to address more memory. Faster and more memory means greater simultaneous support for more avatars, more prims, more scripts, etc. It's a good thing. :D

As far as the grid goes, it should also be a good thing seeing that everything on the sim side of things will be faster.

Price-wise, things shouldn't change too much as (I'm guessing) that the processors in the Class 5 servers should be able to support 64bit, so only an upgrade of OS and possibly motherboards would need replacing. They might charge for more memory over 4GB per server.