32 bit is the only version unfortunately. But I don't remember this being an issue with SCE, even with all settings to the max (as long as your machine can handle it of course!). No plan of a 64 bit, even for AMS that is the following game from Reiza...
64 bit programs can access more memory. It does not mean that a 64 bit program runs twice as fast, the performance is about the same between 32 and 64 bit.
One of the Reiza guys posted recently in their forums to say it would be too much work to adapt AMS to 64 bit but I guess they would have to do this for their new sim as the 4GB limit would be too restricting.