Ah, I figured he just wanted to know what he needed to run it. T'was my understanding he was already informed of the "why", just wanted to know "how".
Just in case, here's something that explains in detail the issue of the 4GB Windows memory limit:
http://www.dansdata.com/askdan00015.htm
*Apart from that, why you need 64-bit is hard to say; there are benefits, but it's highly dependent on the apllication. Some things might even execute slower, and 64-bit applications have a higher memory footprint than 32-bit applications.