The most immediate advantage is the increased memory space which can be addressed, in other words a lot more RAM can be used by the system which in turn benefits in terms of speed of operation, the number of applications and processes which can be running and so on.
It does have disadvantages however, the most prominent probably being the availability of native 64 Bit software etc.
64 bit software for basic stuff (internet use, email and word type documents) is only a small amount quicker 0-10%. For more intensive work it does make a difference - say 20% ripping CDs up to 45% with video authoring work.
You very interestingly say "64bit will allow you to utilise more than the 3gb RAM limit of 32 bit systems"
So if I understand you correctly there is no point in buying a 32 bit machine that has 4gb of RAM. And yet I have seen such machines advertised, or have I? perhaps I am mistaken and in fact what I have seen are in fact 64 bit machines. I also assume there is not point in upgrading a 32bt machine in to 4gb of RAM.