This thread is kind of pointless, it really depends on how much memory you have and what kind of cpu you have to determine the % usage. Everyone is going to be different. Especially with superfetch on, depending on what programs you use, 7 will start prefetching programs or using the disk cache.
What would be better is to just post your benchmarks between the two systems and see which one is faster. There are some stickies on the performance forum for posting memory assessment, etc. Where everyone compares their performance to each other.