Can someone recommend some speakers for my 5.1 system?

[Microman]

I have a $74-$100 center speaker. Rears are just some bookshelf speakers. And front use to be some huge speakers, but replaced them with some little tiny speakers from a set my son won at the bowling alley. I am sure I am not getting the correct balance and sound.

Should I look to replace all 5 at once, or should I just go for the fronts.

Also have a subwoofer from Costco.

Thanks.