actually the ram issue is not dependent on whether the OS is 64 or not
it is also dependent on how a 32 bit OS uses PAE
from what i can remember from Win Serv 2003 (Enterprise as Workstation is 4gb max as with XP and Vista) you can actually address more than 4gb of ram (cant remember the exact # out of my head) since PAE is enabled...
linux (any current distro will do) can address more ram than Win can in 32 bit because Win 32 sets a artificial barrier for compatibility (driver-wise).
that out of the way
@Chappy
yes Windows 2008 R2 (which im loving right now...

) has now began to remove this layer by default and has really taken a good chunk of code that needs to be run (that WOW64 is a really big chunk..

) of course this can be brought back but that now that this layer can be remove sends a message to me and to others that 32 will go...
( even vista still had limited support for NTVDM and has the ability to run 16 bit code while limited its still possible...)
Win64 has eliminated this 16 emulation layer and i believe that until Win128 then they will completely remove the 32 layer (as i see then most apps will be running 64 bit anyways)
the mainstream migration to 64 will take some time (i believe that it was same for the 16 to 32 bit also....)
@maddog: i completely agree....
most users wont see the difference until something that *specifically* requires 64 bit computing (games are getting closer and close for mainstream users) and devs can make good usuage of double floating point (which i cant see that happening)
security and stability options such as given by Windows 64 (such as ASLR and 64 driver signing and blocked kernel patching) are not seen by the mainsteam user as something that would wow them into using 64 bit Win because they have heard *things* that appA doesnt work in win64 and are scared that there appB wont work and so they go with something that "just works".
While i agree that most Compat issues are long gone and those little that are out there are by unsupported apps/hardware that are long gone or where update to be compatible is there, unless they can wow the user by showing them how secure 64 is i dont really see 64 becoming mainstream (also hp defaulting in CTO models (others apply) to 32 bit win does not really help either) and since you dont usually see any performace gains (apart from VM really since *most* apps dont really use more than 2gbs (i did say most as i use Auto cad 2009 and you will see a reason for using 64 auto cad instead...)
until people have a use for that ram and processing power that 64 bit (or even 128 bit...

) i can actually see the cloud stopping mainstream use of these machines as all the processing will be done by servers and since really most users do is go online, check email, maybe play a simple game, write a memo or type a letter these users dont need the extra processing power 64 gives....
i have been only concentrating on 64 in general because that is what most users see...
Win64 really is more secure than Win32 because like i said 64 blocks kernel patching which is a big thing to most....
i actually feel a little more safer to know that Win64 makes it a lot more harder for hackers and virus writers to be able to exploit such bugs in Windows....
i would never dream of running Win32 without antivir (which i do run for my server machine....)
@angryman
i would believe it will take some time ( like i said above) for mainstream to switch...
i especially concur on reasoning with a example of bluray...
if you dont have a HDTV and you have a regular tv and dvd player why get bluray?
i dont really need bluray as DVD's quality is *good* *enough* for me....
and even if i do get HDTV is there really a difference i can see without squinting my eyes at the TV?
(also this is my POV as of now, i still have a VHS player and a bunch of VHS tapes [which include homemade] and until HD cameras (as i do have a dvd Video camera)go down in price i dont really see the incentive for me a home user, which is different from my business POV or Educational POV in which i would say if you have the money go for latest standard out there since your going to look better to an employer (in the case of student) or to an investor (in the case of a business) and while i have used HD cameras like i said most people like me dont really see the incentive (at least those who dont have HDTV (while i do have one i dont see a real difference when watching regular TV and when one of my friends hooks up his bluray player..)....