I just put an order in at my local PC shop for a Windows 7 64bit computer. I now have XP Pro 32-bit.
16GB RAM, SSD for the OS, and a 1TB drive for the data. I figure once prices drop on SSD's I'll get one for data. I also read a LOT about how they fail quickly and can't afford $700 for a huge SSD right now.
I also am getting a hexacore CPU.
So I'm going from a quadcore, 3.2GB RAM machine to a pretty-much up-to-date machine.
I think the biggest benefit will be the CPU and RAM enhancements. That is thanks to the 64-bit OS when it comes to RAM. I think that it is definately worth the money when you consider how cheaper good parts are nowadays compared to a few years ago. When I built the PC I'm currently typing on (which is still not a bad PC at all), it was 2x+ as much money. If you think you need the benefit of 4x more RAM, more stable version of Sonar with large projects, etc... get 64-bit.
Just make sure all your drivers will be happy haha