The 3GB RAM limit isn't a 3GB limit at all. It's 4GB minus the memory required to address your hardware. The more hardware you have, the more memory you lose. Most systems have around the 3.2GB - 3.5GB mark available RAM to the operating system.
Other than that, if you have good hardware and less than 4GB RAM installed you won't receive any regularly tangible benefit from an x64 installation with W7.
Is x64 worth it? Well, yes. Applications are more commonly being written now to utilise the extended RAM capacity. There are however a few apps that won't run on x64, so be careful.