Specs on the speakers are here including size, etc:
http://krksys.com/prodland_rokit.php
I'd easily recommend the RP6, but if you can afford it go with the RP8 for more extended bass response.
They work with any PC, all you need is an RCA -> 1/8" Stereo minijack adapter/Y-cable like this one:
RadioShack.com - Cables, Parts & Connectors: Connectors & connectivity: A/V connectors & adapters: Y-Adapter, Phono Jacks to Stereo 1/8" Plug
They pretty much destroy anything in their pricerange. I have had the RP8 speakers for some time and have not found a match for them anywhere near their price. Their accuracy is stunning, as is their lack of distortion even at intense volumes.