I'm running the x64-bit version of Windows 7 RC as well, and so far I've noticed a definite improvement over Vista. (Use to use Vista Ultimate x32-bit, so maybe it being x64-bit might make some of the difference?) I have 4GB of memory, but am going to get another 4GB for a total of 8GB.
As far as upgrading from Vista to 7, I'd recommend getting the "full" version of the OS software, not the "upgrade" version. If you already have Vista installed on your system and it's been on there for awhile (i.e., you've got a lot of 3rd party software, hardware, drivers, etc. already installed), you could run into problems either installing the upgrade or from the "upgraded" version of Windows 7 itself. If you do get the "upgrade" version, you should do a completely fresh install of Vista, then install the upgrade immediately afterward. That would give the best chance of the "upgrade" working properly.
Lastly, since Windows 7 comes with both the x32 & x64-bit versions on them, can you upgrade from a full x32-bit OS to the x64-bit version of Windows 7 or will it only allow you to upgrade to the same-bit version? I'd imagine the latter than the former.