Luc, Music stored on hard drive has no timing - only data. As long as interface is faster than required data rate it will be OK. Data rate required for standard 16/44.1 CD is 1.4Mb/s while USB 2.0 is running at about 240Mb/s (wasting about 50% of 480Mb/s on protocol). It all goes thru buffers anyway.
Great thing about SSD for music server other than lack of noise and heat (that Xti16 already mentioned) is reliability. SSD has limited number of writes but unlimited number of reads and no mechanical components. Firewire being expansion bus (vs USB being peripheral bus) is that it has sustained rate of 400Mb/s and doesn't require main processor for transfers the way USB does. I use it because my server is also my main computer, but with dedicated server it doesn't even matter - anything will do.
