Yes, i Agree that now-a-days software decoders a very good, and if you don't really care about just haveing normal sound quality/separation on 5.1 audio, then software decoding is definitly the way to go (it also lets you then buy a cheaper set of speakers, as you don't need and amp.)
But if you seriously want good 5.1 dolby digital (which doesn't matter that much as the Audigy 2 will be able to decode that) but more importantly 5.1 DTS, a hardware is much better than software (as in most cases).
The clarity/crispness and mostly the sound separation and the overall sound field will be much better and of higher quailty. You will also get the full benefits of the higher encoded DTS.
And i say this in experience, as i first of all, have a home cinema (with a big sony amp.) and then on my PC i've got the Creative Decoder and Speakers and i also use Power DVD 4.0 to play DVD's, and switching between hardware and software decoding is a noticeable difference (note: going from software to hardware is not, bad to good, it's good to great!)
But all up it really depends on how much you are willing to pay for the sound system (for me, i though i might as well go nearly full out and buy a decent set of speakers). But the cheapest option for good DTS and DD is the software decoding option along with a Huge range of excellent, inexpensive 5.1 speakers.
Or there is the more expensive alternative where you go out and buy a set of 5.1 Digital speakers ie. Yamaha, Creative, Logitech, Klipsch (getting to more proffesional audio) or finaly Boston Acoustics (seriouse 5.1 PC Home Theater) You could even go out and buy a $1000 dollar Amp and $2000 worth of speakers, and hook that all up (as i've know some to do) and have a Really Good sound system, thogh personally i think that's a bit much for a PC
)
i'm sorry though if i rambled on, but i hope i helped in some small way
)
cheers
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