Hmm, I didnt notice a big difference on my X-Fi between high and ultra sound settings.
It handles more sounds at one time but the sound quality itself is still 16/48 sampling.
Comparing the X-Fi to your onboard sound the they difference is very noticable and you will likely be limited by your speaker setup.
Since you dont ahev Xram you will want to limit your voices to 64 instead of the full 128 that the X-Fi is capable of.