Turtle Beach use to be a good sound card. They are not any longer. They simply repackage other low end cards made by other companies now.
There are only a couple of companies making DixrectX compatible sound cards these days.
Creative Labs and Auzentech (sp?). Both have pluses and minuses. If you have any thoughts about moving towards Vista, Creative is probably the best bet. They have resurrected the DirectSound 3D API, which Microsoft decided to kill in DX10.