you'll want to hook up the tape outs to the "line" input of your soundcard as said before. then record using a 2-track recording program like Sound Forge:
before you start, ABSOLUTELY IMPORTANT: if using something like a soundblaster, make sure that all inputs other than the "line" input are muted! otherwise you'll get more noise than you should.
1. record at stereo 44.1khz/16bit
2. zero the clip meters on the software
3. do one pass with the tape running through the entire track watching the meters and the clip light. if the clip light goes on, then redo with a lower volume.
4. look for a couple db of headroom. if your meters are too close to 0db, restart with a lower volume.
5. now you're ready to record. hit the red record button on your software, and hit play on the tape. if after the recording is done and the clip lights went on, then redo.
6. normalize to 0db if not using compression. if your tape -> computer transfer has some random/erratic spikes, use compression--Sound Forge has compression built into its normalize function. i think it recommends normalizing to 10db with compression. i dunno... whatever it recommends is good.
7. it's done. save to the format of choice. i would save to non-compressed .wav as a backup, then save to .mp3 or whatever.
hope this helps.