I have a two-year-old:

Systemax Venture B805 RTS PC
Pent D Proc 2.66 GHz Dual Core
1024 Memory
250 GB 7200 RPM Hard Drive
XP Professional

I have the GARDEN variety sound card, without upgrade, supplied with the PC.

I am into audio editing and CD production of stereo sound from a clean, high quality source.

However, the signal-to-noise ratio of the A to D on the stereo line input is only marginal, not nearly as good as the incoming audio. The right channel is about 20 db worse than the left.

I am resigned to purchase a better sound card. I don't care about gaming or multi channel (4, 5, or 7) outputs, only a clean stereo input to the A/D.

What sound card would meet my requirements? What are you using to measure noise levels? Most of the noise is not from the A/C converters. It is on the line coming into the sound device. You could try a USB external sound device, but some of these are not much better. You could get a professional card, but these are very top-end PRICEY things.
Here is just one:
http://www.proaudio.com/product_info.php?cPath=475_377&products_id=901
At almost $200 that card is in the low GROUP for professional use. It has over 100db range, so you could expect better than 70db S/n and still have 30db HEAD room. Too much money for me, so I yell into the microphone! Sometimes I can get 40 db S/N.Thank you for your prompt reply.

I can SEE the noise floor on the input metering with the LINE IN cable disconnected.

Will investigate further. Thanks.Then you have a very poor sound card. Even one of the $25 sound Blaster cards do better. More often the Mic in that has a high noise level and most of the noise is pickup on the cable. That is the problem I have.