For the effect you need to build the system with one, then the other and then run benchmarks only changing that value. For most of us the effect is small. Plus for gaming I can put the dollars for higher FSB into a better GPU and could declare it the winner for gaming. However it might not transcode video (video editing!) as fast.
What's the goal?
I am debating between two new systems and could use some help. One has a Intel Pentium D 935 3.2GHz with 800FSB, the other ($60.more) has a Intel Pentium E6550 2.3GHz with 1333FSB. I am not a power user or a gamer but am looking for reliability for general home use. All other factors equal is the faster FSB worth dropping 1GHz in speed and $60. more?