I finally took the plunge. Now I get to use 4GB out of 4GB except that the bare minimum I seem to be able to get Vista x64 down to is a 1.2GB footprint. And that's after hours and hours of experimentation and disabling some visual enhancements, though I feel no loss there and am experiencing a significantly reduced sense of loss.
Now I'm happy to be able to test on x64 virtual images using VMWare's Workstation, I'm afraid I may need to buy four 2GB sticks of RAM now. Despite the fact that the additional memory is available now, the larger footprint nearly wipes out the gain.
And that's without running any significant applications, except IE, which is quite a memory hog. I guess the old 640K upper limit days are over.
Yes, RAM is cheap. A quick check on Newegg.com and I found 8GB (4 x 2GB DDR2 800) for $174. I can't even buy three tanks of gas for my SUV for that.