Even a $40 sound card will do a good job with audio cassettes as the noise from the cassette is much higher than the noise floor of a recent but cheapo sound card. Use a good quality cassette deck for the source, and keep an eye on the levels into your sound card. Make sure your hard drive is defragmented and that you have lots o room on it. Do a 60 sec recording of nothing and note the disk space it uses. Figure out how much run time you have on your hard drive from that and then leave at least half the drive space available for editing. Pump those cassettes in and out leaving a gap where the tapes start and stop until you run out of half of your drive space. At that point, go through the tunes one by one copying the particular track and then pasting it into a new file that you save.
If you have problems with an older sound card introducing noise, make sure that you don't have any drive cables rubbing on the sound card or that the video card isn't sitting right next to the video card.