The current version as of yesterday is 0.5.2
Get the 32 bit client.
If you have a 64 bit processor and a 64 bit version of your operating system then you can run the 64 bit version however you probably will not notice any improvement.
Now, to answer your question about 32 bit versus 64 bit.
32 bit computers can only access up to 4 gigabytes of ram memory.
To get past this limitation *efficiently* 64 bit processors were introduced. These 64 bit processors are also capable of efficiently handling large numbers but I believe that's
Not that important now that accelerator cards are commonly used.
Back In the day when 16 bit processors were common and graphics accelerators were not, graphical data could be processed faster by using 32 bit code. ( Fixedpoint math )
So *upgrading* to a 32 bit processor and getting an os or a os library that utilzed the 32bit registers was a must.