Equipment suggestions
 

I have been making a ton of music on Pro Tools 9 and FL studio. Making music is one thing and actually mastering is another thing all together. I bought an ASUS Xonar STX sound card for something better than my onbaord audio but i am wondering for the purpose of recording my projects do i need anything better than that? I am new to mastering and need some suggestions on gear i will need for my studio at home. I already got some decent Mackie MR8's. How do i add some warmth(analog) to my sound on my fully digital setup when it come to sampling/recording?

Also I was wanting to find a good USB sound card for my laptop to reduce latency with Pro Tools and just have something better than onboard with my Sony VAIO. Any suggestions?