External sound card help

I did some research on the M-Audio Fast Track USB that Chris suggested and this item and it has major latency issues.

I have programs on my pc that I have used to programmed full musical arrangements with, I need to be able to hear that in real time and play over it while still being able to hear my guitar, all the while having the ability to record it with other software that I have.

So, I guess that my next question is, can someone explain to me how an external sound-card works and could this be the solution to my problem?

Hello mate. As this is not really my comfort zone, I hope someone with more experience with external sound cards can help, but his might be interesting reading for you:

Music On My PC - How To Connect a Microphone To Your PC Computer Laptop or Mac (Mic/Mike)

DONT BE MISLEAD BY THE LINK WORDING, ITS NOT REALLY ALL ABOUT MIC CONNECTION.
The site itself seems to be a mineful of information about pc music.