I am running Win 7 RC x64 with 8GB memory on a Core 2 Duo CPU, and I have one of those little applets that tracks CPU and memory on the desktop. I seldom see it go past 30% of memory used, and CPU is always under 30%. My app profile looks similar to yours, I don't do any app dev or use any x64 apps on this site, nothing intensive like movie editing etc. Even when it runs backup it's around 30% CPU.
My experience and with speaking to others, you can't even really tell when something is in WOW64 vs. a true 64 bit app. I have run Firefox x64 and it feels the same (except that a bunch of add-ons don't run in x64
I suspect part of your CPU use is due to the memory you have, there is probably a little more swapping etc. going on. Adding memory will help the CPU usage.