eaerken,
You haven't said exactly what model of subwoofer you have. That makes a difference as to how you connect other speakers to it. There are way too many different designs for people to guess.
However, if you want quality sound from the PC, things probably will be easier to interconnect if you get a receiver that has an optical digital audio input. Some audio cards produce very high quality analog audio, but motherboard audio chipsets usually do not.
I dunno what speakers are easily available in your part of the world, so I'm not sure what to recommend. Listening to them yourself at a local A/V store would be best, if that's possible.