External Sound Card for Audio Analysis?

I finally decided to go with a laptop (PC) to analyze preamp and power amp performance. The only issue I am facing now is how to get a good audio signal into and out of the laptop. The laptop only has a combo headphone/mic jack which I doubt is very good in the way of sound quality and I think only has mono input. Therefore, I believe that the best way to go is to get an external sound card that will plug into a USB port.

Not being familiar with sound cards, I am looking for recommendations for an external sound card that will output stereo signals from the audio analyzer software and provide stereo inputs into the laptop so I can analyze the output from stereo preamps and power amps using a dummy load. That means that the sound card input will need to be able to accept 600 ohms from a preamp output and 8 ohms from a power amp output.

I don't have tons of money, but I want something that will give me reasonably accurate results for a reasonable price.

Suggestions?

Thanks,

Scott