Hello:
I run duel boot, Windows XP Pro x64 for my main OS. And I have Windows 7 RC1 x64 for secondary and messing around reasons. For me, I do see the differences. And, I can see the differences, because I have been in computing since the old DOS days and my first computer was an IBM compatible XT machine. Especially in booting up, so much faster. I run 6gb 667's in my machine now. But I am going to be upgrading to faster ram and 8gb too and a new cpu shortly. Since Ram is cheap now. Where the extra ram really comes in handy, is if you do a lot of photo and video editing or CADD designing.
And computer programs are getting bigger all the time, it is nice to ready for it. (Ram wise).