SRS Audio Sandbox is just a more advanced software DSP than the equalizer that comes with your soundblaster and really you want to avoid relying on heavily equalizing your audio to try to get it to sound better - in fact you should never need to equalize your music if your hardware is up to the task.
The X-Fi Xtreme Music is an okay card, but IMO better can be had for the money in terms of hardware quality. Much of the supposed quality of the X-Fi range of cards falls down to Creative very clever DSP effects, which as controversial as this view is to some people, is actually detrimental to the music and is giving you a false impression of quality. Some people also seem to suffer from compatibility issues and popping/crackling from these cards.
Your speakers are pretty good in terms of PC audio, a decent sound card should provide a rich enough sound without needing to be tweaking the EQ all the time.
Personally Id be looking at an
M-Audio Revolution 7.1 in your situation. They also do a 5.1 version if you didnt want to stretch your budget that far.