Go with the socket 939 model. It'll probably be a Venice core as opposed to the older Newcastle core. Venice core is more efficient and adds an extra instruction set. Also, socket 939 is a bit more future-proof as that's the direction AMD are going with their desktop CPUs at the moment.
FSB is short for Front Side Bus. It's the clock rate at which the CPU communicates with other devices in the system, most importantly the memory. The faster the clock speed of the FSB, the more data can be moved back and forth in a given interval of time (actually that's an oversimplification, but for the purposes of your comparison it's enough). So you actually do get slightly better performance with the 3500+ processor despite the core clock speed being lower than the 3400+