need rear surrond speakers

[mpk200]

i have a yamaha rxv420rds amp a pair of celstion f2 floor standers for front and a gale centre.

at the moment i am using my old mission 762 from my hifi for rears but now have £100 to spend on rears any help please .
do i need a pair as good as the front ones or not

i would probonly like smaller speakers at the rear to blend in

thanks

mike