Forgive me if this has already been posted, but here is a very interesting article about the benefits of 64-bit and how they'll be of use in the upcoming 10.6 Snow Leopard.
Especially note the "Great PC RAM Swindle" section.
Part 1: http://www.appleinsider.com/articles/08/08/26/road_to_mac_os_x_10_6_snow_leopard_64_bits.html
Part 2: http://www.appleinsider.com/articles/08/09/03/road_to_mac_os_x_snow_leopard_64_bits_santa_rosa_and_the_great_pc_swindle.html
Especially note the "Great PC RAM Swindle" section.
Part 1: http://www.appleinsider.com/articles/08/08/26/road_to_mac_os_x_10_6_snow_leopard_64_bits.html
Part 2: http://www.appleinsider.com/articles/08/09/03/road_to_mac_os_x_snow_leopard_64_bits_santa_rosa_and_the_great_pc_swindle.html