Refer to Gannu/Sangram aka Cracky's guide for the same.
tirthankar said:2.1: Asus Xonar DX + Altec Lansing MX5021
5.1: Asus Xonar DX + Logitech X540 (I think iGo bought it, and he is quite happy with it).
However, SQ wise the MX5021 would be leagues ahead of X-540. get the latter only if you require surround (movies with 5.1 sound and games). No use if you only watch dvdrips with 2-channel sound.
The latest MXs are sometimes said to have build quality issues. just check them out physically before buying.
Let us know what you opt for.