It depends on how you look at it. In absolute terms, the standard QPI buss is equal in total/theoretical bandwidth with the fastest (1600) FSB. The difference is that FSB uses the same lanes for traffic in both directions
(these can NOT both be active at the same instant), whereas QPI has dedicated lanes for each. So given the artificial situation where we have perfect utilization in both directions, with all lanes fully saturated, at all times, the slowest implementation of QPI is theoretically twice as fast as the fastest (*1600) FSB. That never happens in real life, though.
QPI also has an advantage of higher clock speed and lower latency
(so in some few artificially created cases it could theoretically 'feel' more responsive to the end user). There is also the indirect architectural advantage of having the L3 cache on the processor on a QPI equipped system. With a FSB, communication between Processor cores has to be routed through the FSB. In a (Nehalem) that is handled by the *much* faster L3, and any bandwidth used in this manner never hits the QPI.
http://www.hardwaresecrets.com/article/610
AMD's Hypertransport has the same architetural advantages, but less overall bandwidth.
"In Practice", none of this really matters on a desktop computer. On the Server side, it may or may not matter - depending on the type of workload.