[Definity]

Hi i was reading PAFO thread about how he could make his mix's more smooth. This got me thinking about the steps people would take to making a more professional mix, i am a self taught muscian and producer for 4 years and would love to know how to get a higher standard of mastering/mixing from the pros equitment/methods and so on. I have now found out about DAC and how they can higlight certain things that dont seam to be present straight away in a mix, also about how important it is to have a decent set of monitors. But what about sound cards/audio interfaces i have got a 24bit 192khz soundcard like most, but leaves me thinking what is the differance between the 1500 quid inerfaces and the 150 quid ones? are some actual better quality even though they are the same bit and sample rate? i was hoping some pointers in the right direction should help me increase the quality of my tracks so is there any websites, books, tutorials, personal knowledge or equitment which could help me would be a MASSIVE help to me and others!
Thanks
Def