I did it my self and i dont really notice much of a speed diffrence ( I put a 80GB 5400rpm 8MB Cache Western Digital in) just loads more space, mines a bit noiser due to the higher rpm and the fan in the ibook spins up a bit, never used to notice it before, but then I havnt been running it on a stand like I do normally. I don't think its going to give you a huge speed boost, unless your do a lot of disk intensive stuff, I'd say throw more RAM at it first. I also belive that if you do it yourself you don't void the warranty but they wont cover any damage you do, or the new component, however I would have thought that once they worked out that you opened the case their going to blaim eveything on you anyway. If you can think of any benchmarks I could run on my ibook to try and work out if it acctually faster I could give them a go if you want.
I break stuff.