But what do you need more than 3GB of RAM for ?
In addition to the things discussed above by others above, there are two additional issues to consider:
1. Not only does 32-bit Windows have a total addressable RAM limit of 4GB (usually reduced by onboard devices further), but it has a per-process limit of 2GB. This is, honestly, quite easy to hit. But the problem becomes much worse if you are doing
any multitasking with your system (including system tray processes). So, Windows itself often uses around 1GB of RAM while running. So, if you're starting at 3.5GB available, that takes you down to 2.5 available. Then, you run some system tray processes and you're down to below 2GB available. In this instance, each application has a 2GB cap, but you don't have that much available, so if you launch two memory-hungry processes at the same time? It swaps to disk. Swapping to disk is TERRIBLE and should be avoided at all costs (even with the swap file on an SSD, the memory access latencies are an order-of-magnitude higher than accessing RAM).
Many applications are happy to use ~2GB, not just "pro" apps. Microsoft Word and Outlook will certainly do it. Heck, load a bunch of tabs in your browser that contain Flash, and look at the memory usage.
It is easy to get into a RAM limited state on a 32-bit version of Windows. Even if no single application uses 2GB, if you run 3-4 of them at a time (maybe MC in the front, in Theater View, with a big library, while you have a browser open with 10 tabs, and some dumb weather-checking tray application that loads WAY more than it needs to...). The problem is aggregate, not single applications.
2. Even if #1 never happens, Windows 7 has a great feature called SuperFetch. This is a predictive algorithm that caches files you are likely to access in RAM before they are accessed. This can have a fairly dramatic impact on everyday system performance (and it also helps with things like reboot speeds fairly dramatically). It comes down to this: Windows 7 will use the RAM you give it. If not for actually running applications, then for SuperFetch. Is 16GB total probably overkill for regular HTPC use for most people? Certainly. Is 6 or 8GB? Not at all. Even if everything you run on a daily basis never uses more than a few hundred MB of RAM (which is unlikely), SuperFetch will use the remainder of the available RAM to cache disk reads and writes, which is the primary "cause" of latency with modern PCs.
PS. If you've read any guides that recommend disabling SuperFetch, they do NOT know what the heck they're talking about. SuperFetch is a Very Good Feature of Windows 7, and provides easy-to-demonstrate performance enhancements. Many of the "optimization" guides out there that recommend disabling services and whatnot are WAY off their marks. Even the "good" sources of information, don't provide a lot of systematic evidence of improvement from their procedures, and can often cause more trouble than good if you aren't extremely judicious in what you follow.
With a modern Windows 7 PC, the two best things you can upgrade to improve everyday performance are (in order):
1. Get a SSD.
2. If you have less than 4GB of RAM, upgrade to 8GB.
If you have the 32-bit version of the OS, you can't do #2, and this is the cheapest option by far (since RAM is absurdly cheap right now). And, since there is really no reason to worry about it otherwise, why limit yourself?
I've been using a 64-bit build of Windows 7 since it first came out in the Betas, and I've never, ever, hit an application that didn't work right on Windows 7 64-bit which worked right on Win7 32-bit. Like I said above, there are examples, but they are edge-cases. Even if you hit these, in most cases, it is because you are tying to use something incredibly old and abandoned (which could, very well, have its own set of issues, especially security-related).
(An ancient version of Quicken is a perfect example. You really want to keep all of your financial information in an application that hasn't been updated in 12+ years? An application that has system services and an online component? Really? You've read that there have been some changes in security procedures on Windows since Windows ME, right?)