Yep, the front are 4.5" and rear 4". I bought my rear speakers from J&M and they are both water proof and sound excellent. I think it was about $50 for the pair. There's also a wiring harness made for the IS if you're interested I could dig up a link for you.
As for the amp, there's no reason you couldn't install one. If you do, you'll need aftermarket speakers because the OEM set aren't made to handle the extra watts.
I mostly use my headset speakers and have my cycle speakers turned off. I prefer this because it lets me put in ear plugs to block out wind noise and crank up the headset speakers to listen to music. I know MP does this as well since I stole the idea from him!
