One thing first. We are taling here about Dolby Digital, right ?
Well in that case the Soundcard must have an input for either an optical or coaxial cable... The only way to get a good quality DIGITAL Sound out of your PS2 is via the optical out !!!
If the Soundcard supports this, then it´s OK... It´s not the best solution, but for the beginning it will do the job.
Second: If you just want surround sound (not Digital) than you just buy a soundcard that can decode DOLBY SURROUND Sound... You just ´plug then the right and left cinch out from the PS2 AV cable into the soundcard and of you go... BUT this will bring you no Dolby Digital or DTS Sound !!!
I´m not so into this Soundcard Stuff, but for a good setup here you have to pay the same money like for a receiver and speakers... for less quality - you pay less !
Last thing: Remember that the Dolby Digital and DTS Sound is SUPERB and once you´ve heared it you don´t want any other... You must have the PS2 (you have it, goooood boy
), a Dolby Digital/DTS Receiver, an optical Cable and 5 speakers (+ Sub)...
I would go for a receiver and speakers, you then don´t have to run you PC all the time and it´s much better Sound - you just can´t get the Power out of a Soundcard...
Hope that helps a bit...