Currently my big machine doesnt even have a soundcard, it has a Realtek "audio" thing probably on the motherboard, not sure.
 
it sounds good and I have nice speakers attached... but I wonder if I could get something better, more "flat" for mixing
 
any suggestions would be helpful

Find any post by Beagle and click his link for sound card/audio interface recommendations.
Whatever you decide on, make sure it has ASIO drivers for your operating system and all the ins and outs you need.
I like Focusrite.

It depends upon how much you have to spend.  I'd go for Burl, but it starts at $2500 w/o any ADDA.
 
Back in the real world, it depends upon money (again), and how many in/outs you need.  Most of the lower-cost cards are about the same, sound-wise, and price-wise for similiar capabilities.  Roland, Focusrite and Stienberg/Yamaha all seem to work for most people and are solid.  I used to suggest the TC Konnekt 6, which was a table top unit w/ a large volume knob, one preamp and stereo in/out.  But it is FW, which is dying, however,  if you have a suitable computer or card it works fine, esp. w/ the newest drivers (64-bit).  $200, and capable of pro sound.
 
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