If you are running a mac as your source, you could get a very decent used receiver on this site, like a Denon or an Onkyo, and run an optical cable from the mac to the digital input on the receiver. This could give you pretty decent watts-per-channel for your passive speakers.
If you are running a PC laptop as your source, you need an external, USB-based DAC. This decreases the amount of money you can spend on amplification, which is very important with passive speakers. I suggest you get your hands on some powered speakers in this case, Audioengine is a good place to start for this, and they have a decent USB DAC too. Quad also has made powered speakers that even have an internal USB DAC, I have not heard them, and they are out of production, but you can still find them used.
If you are using a PC desktop as a source, you should be able to get your hands on a decent sound card for not too much money, if you don't want to go with a USB DAC. I know nothing about the sound card route.
If you are running a PC laptop as your source, you need an external, USB-based DAC. This decreases the amount of money you can spend on amplification, which is very important with passive speakers. I suggest you get your hands on some powered speakers in this case, Audioengine is a good place to start for this, and they have a decent USB DAC too. Quad also has made powered speakers that even have an internal USB DAC, I have not heard them, and they are out of production, but you can still find them used.
If you are using a PC desktop as a source, you should be able to get your hands on a decent sound card for not too much money, if you don't want to go with a USB DAC. I know nothing about the sound card route.