wat ? creative speakers crack at 50% of the volume ? try the high end ones bro .. moreover starting from 5200 and above , all the models have the beat cancel technology , helps to higher the volume a bit ,
Regarding the query , first of all you need to have a good quality speakers to get high volume , But since you already own the speakers , consider a good sound card , go for the SB live 5.1 , it will reproduce much more sound than the onboard solution , and lesser CPU overhead,next to negligible .. Intel may call it high definition or whatever ,
One more tip i wanted to give you is that , you may choose to keep the sub woofer in an enclosed wooden area or a closed area with some closed space for the extra beat , and that's all you can do about it , i am saying this because i had the 4.1 and the 5.1 in the past , a lot of experiment has been done by me

, I even tried combining 41 and the 5.1 to get a 9.1 solution ,but it was all crap ,
Anyways thats pretty much about it !