Any 6x9 speaker should work just fine, you don't need to replace out your front speakers but it's probably a good idea if you have the extra cash... I'd recommend a pair of 6x9 Pioneers, I have the same exact ones as seen in the link below for my front speakers (6x8) and they have excellent bass, and clarity...
"http://signature.crutchfield.com/s_130TSD902P/Pioneer-Premier-TS-D902P.html?showAll=Y&tp=91"