Probably a Asus Xonar DX....it has some serious praising over the net,but wait for gurus who may show you some more light.
You didn't mention your budget,
I'm also X-540 user using Intel HD 5.1 onbard.
+1 for ASUS Xonar DX. Though it exceeds your budget, it'll cost you arounf 3.7k. But it's worth that value. Otherwise look at Creative cards.