6-8GB is the sweet spot for Windows Vista/7 64-bit with most current desktop applications, imho. I have 12GB and 16GB machines running 7, and they don't feel significantly faster than those running half that amount for the most part (and they spend much more time prefetching after bootup).
4GB of RAM is still a very reasonable size, and makes sense if you prefer using a 32-bit OS. I find 2GB fairly atrocious with higher iterations of Windows Vista/7 on the other hand, but this can still work great with stripped-down variants (Starter and Home Basic).
With today's fast processors, I'd happily go down a notch or two in speed grade in most cases if that'd translate to more usable memory (upto a certain point, of course) -- unless the PC would be dedicated for processing-intensive tasks such as data compression.