those are 8ohm and 100w rated speakers which means that you will need an amplifier capable of that rating or perhaps just a tad more.
this could be either just an amplifier or a dac+amp. the first would use your soundcard and an analog (3.5mm) connection while the second would likely use either usb or optical spdif and will pretty much bypass your soundcard.
normal amplifiers are available in many quality grades from very cheap (about $10) to models which are very expensive (over $500). it would be best to match the quality of the amp to the quality of the speakers. if you wanted to go with an external dac (digital analog converter... basically what is inside a soundcard) and amp (also in a soundcard) combination then you have three options. you could buy seperate dac and amp units, you can buy a combined dac+amp unit or you can buy something like a reciever/stereo amp which often have many inputs and in the case of a stereo amp they normally have a cd player built in.
if this was my own setup... i'd probably use a stereo receiver since i have multiple inputs but if i only needed to use it with pc then i'd go with either an external dac+amp combo or a hifi system amp with cd player.
if you want specific recommendations for models i would highly advise you to stick your nose in over at www.head-fi.org or www.avsforum.com since those sites are primarily audio oriented and will be able to give you a better listing of products.