Here's a good read about the subject.
I've seen his blog before, but no one can come up with conclusions till like a year from now where people can test the memory usage for themselves, maybe even get some phoronix benchmarks.
for now am using 32bit on my 3gb ram laptop and hope to test x32 in the future.
Unity is the slowest desktop environment I've ever used, just like Ubuntu Software Center. Even on a high-end machine the "lens" takes a second or two to load, USC takes forever to load, as does switching between multiple open windows of the same program zoom effect thing. Sad that you can no longer recommend Ubuntu as a cure for older computers (older distros don't count, the newest version of Firefox and other programs are needed). While I agree that DEs can afford to look nicer now days, not having the ability to "turn off" (drop to Metacity or whatever) the shinies like you can on Windows 7 is fail since they are dropping Unity 2D. You can't compare Win7 to Unity though because even with the desktop shinies on, Win7 is infinitely times faster since Win7's start menu opens instantly. What part of pre-loading all the DE's graphics libraries doesn't Unity understand? Maybe when Unity isn't such a bloated pig will more users actually like it.
I can still feel the lag in win7's menu too, it's not perfect nor instant.
But it's certainly faster than Unity on comparable hw, agreed.
I switched to Xubuntu right before Unity became Ubuntu's default.
The 32 bit architecture should simply be avoided, unless the hardware demands it, or you have less than 2g of ram. If you are in that scenario, you probably need to use Xubuntu or Lubuntu anyway.
I still hope someone from Canonical clears this up.