I don't know any of the technical info for you CLK. But if it's a cabriolet 6 speakers, and if it's a coupe, then 8 speakers.
You cannot change the speakers if you intend on keeping the factory head unit and amp. It will not work properly, and you could blow them very easily. I believe Mercedes uses 4 ohm speakers, and the usual is 6 or 2 ohms, the 4 being very hard to find. (Correct me if I am wrong anyone)

If you want more punch, I would suggest adding an amp and sub. I have a 400 watt amp (bypassing original amp) and a 10 inch sub. It sounds great now!

__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)