Ohms!! my head hurts!

[tnuthals]

Ok...i've been digging through information about speaker impedance for awhile and now my head hurts. I have a couple questions/hypothetical situations andI need you folks to set me straight.

I'm planning on getting a pair of Polk Lsi15s (rated as 4ohms) to be my new fronts. I currently have a yamaha 663 as my receiver.

First, according to the manual, I can change the speaker impedence setting to 6 ohms.....thus allowing me to use fronts that are rated at least 4 ohms...as long as my center and two surrounds are 8 ohms (which they currently are).
Is this assumption correct??

Second, If i wanted to upgrad my center and rears to lsi9s and an lsic, I would not be able to use my receiver....unless i got an amp?

Should I just invest in an amp now?
Should I start shopping for a new receiver?

Help, thanks.