my father gave me his old receiver and 4 speakers.

the receiver is an onkyo tx-ds575 which supports 5.1 channels, dts, dolby digital, and dolby pro logic (1 only i assume). the instruction manual says it supports 70 watts minimum at 8ohms (usa models) and 100 watts minimum at 6ohms. (european models, din???) i don't know what that means exactly. so i should use 6ohm speakers for more watts i assume?

the speakers are made by denon. the 2 front speakers are 100watts at 6ohms and the 2 rear speakers are 30watts at 6ohms.

i don't have a center speaker yet or a subwoofer. what should i get that will go with my stuff based on those above specs? i don't understand this ohm stuff and i don't know what kind of subwoofer would be appropriate for my receiver (watts, etc?).

First off, I really like the Onkyo 575. I own a similiar Onkyo model. You are correct in assuming it only has standard Dolby Pro Logic though.

Most modern subs are active or powered. Therefore they don't use the amp section of your receiver.

Subs range from $100 to $5000. Two important factors are how large your room is and how loud you plan on playing your movies/music/games/etc.

As for the center channel, you want the sound to match your front speakers. I recommend seeing if Denon makes a center channel in the same line of speakers.