Reply to Soundcard for KRK Rokit Moniters?

*Warning , ultra noob post*

So , I'm looking to step up my ten dollar wal-mart speakers for a bigger , better , louder setup .

I've been looking around for a while & decided that the KRK Rokit 5 matches my price and what I'm going to be using it for , which is a DJ'ing and running out of Ableton .

I have a friend who got the Rokit 6 but didn't have a soundcard , therefore could not use them! He eventually got one , so its fine now , but I was wondering if and how I should use a soundcard in my setup

I own an S2 , but I dont believe its enough to run to the KRK's ?

I want to know a few things

1) Do I need a soundcard?
2) What soundcard do you recommend?

I would also want something that I could use with a friend running from their computer , and as well in the future with Pioneer's as I seek to expand my setup !

Thanks so much !
Swede Dreams