I haven't finished reading this entire thread, but from what I have been reading, I definitely would like to build a new PC with all 64-bit hardware and take advantage of the extra RAM. However, I still may hold on to my 32-bit XP version for certain reasons.
For my future HTPC build, I could go ahead and consider going 64-bit but if its primary purpose is to pretty much stream movies and content over my home network, you usually wouldn't need a 64-bit system to accomplish that as a high-powered 32-bit system can do the same job quite well. I still have a full-tower PC I built back in early 2004 that has the P4 3.2GHz HT processor (which is definitely no slouch) that I primarily built for video editing. Won't support a 64-bit OS if I moved to Win7 because of the older hardware but I can easily convert it to an HTPC and with 2GB of RAM I have in it currently (out of 4), it will have no problem running as a successful HTPC, with only changing out some hardware components (like upgrading to a 1.5TB HD from the 3 old 150GB HD's I installed at that time). And XP is stable enough to handle that with no problem, though 32-bit Win7 will run just fine as well.
My next video editing rig I plan to build will be an 64-bit system since the software I use for editing (Adobe Master CS4) will soon be 64-bit only, plus will make encoding tasks much easier in terms of rendering and such.
However, I also do audio editing on the side and I have the EM-U 1616m soundcard which from what I heard Creative has yet to create any drivers for Win7, let alone 64-bit. They claim that the Vista drivers will work fine, but I am not convinced. Until I can test and confirm this or until Win7 drivers (and 64-bit) for my soundcard are available, I will stay with 32-bit XP for my laptop which I do my audio work on. In the meantime I have a dual-boot setup where I can test the hardware on Win7 before I consider commitment to change.
I am not a PC gamer so it makes my choices easier.