Ferris-
Great choice! I had a 2000 Civic Si from January 2001 till Dec 27, 2001 (it got stolen, and they never found it). I couldn't find another in my area so I just bought an EX.
Anyway, this car is downright fast even when bone stock. Sounds like you already have some mods on so it should be a screamer. If you're still wondering how to get those rear speakers out, you have to take the rear shelf tray off in order to access the screws that hold the speakers down. Then, simply remove the connector from the back of the speaker. The proper (service manual) method to remove the rear shelf is to remove the rear seat backs and cushion, then those two large plastic side panels, then the shelf. However, most people just fold the seat down and try to just pop the shelf out (be warned, it's a little tricky this way).
In my Si, I had an AEM cold air intake, AEM Tru-Power pulleys, AEM adjustable cam gears, DC Sports 4-2-1 header, and a Powercore high-flow cat. All of these mods netted me 21 hp to the wheels, which is definitely noticable! The car ran 15.28 in the 400 m (1/4 mile).
Have fun with your new Si (especially over 5400 r/min)!