download them all....ask questions later  :D

Every Rybka version comes in 32 and 64bit flavours.  To get the 64bit version working, you need a system that supports 64bit OS such as Win XP Pro 64bit ... that would require a CPU that is also 64bit capable.  Any system will be able to run the 32bit version.  The 64bit version will run almost 60% faster and thus is quite a bit stronger.  With regards to the LK and non LK versions, the LK is supposed to add some extra knowledge and has some different evaluations ... some which I personally like and others I dislike.  For example it evaluates heavily in favour of a bishop instead of 3 pawns in an ending ... this can backfire dramatically depending on the position.  The new upcoming Rybka's will all have some sort of LK built in by default.