Oh man, this horse has been beaten to death, so much so that it's not even recognizable anymore! You can choose to read long posts after posts by noobs and sages but it really does boil down to very simple facts which is nearly foolish to even contradict these days. To sum all of it up - there's not one single valid reason to stick with 32 bit when building a new system. None at all! But there are many reasons, some extremely practical, some a little more obscure, to go with 64 bits. The most obvious - your CPU was designed to shine on a 64 bit OS, and W7 was meant to shine on systems with lots of RAM. By lots, I mean, minimum 4 gigs; ideally more. With a 32 bit OS, you are capped at 4 gigs, and typically even with that much in your system, only about 75% of your ram is even accessible to the OS.
The argument against 64 bit shouldn't ever be about technological benefits, but rather consumer needs and perceptions. The idea that a PC used for nothing more than messenger, facebook and XXX sites rarely needs that much horsepower, but that's really just silly. All those people who say this, are the first ones to complain about slow PC performance, and they are typically the type to panic and rush over to Bestbuy and have qeeksquad "diagnose" their systems. With the cost of that 'diagnostic', they could have easily gotten the extra RAM, to take full advantage of a x64 bit system and not have to complain about the slow performance of their system a month after they buy their computers. I know this sounds a little subjective, but anyone who builds systems, or works in retail selling PCs knows exactly what I'm talking about.