In my opinion, the easiest and hassle-free way to go is to first compile & record the songs you want into audio cds(assuming you have a cd burner....nero is perfect for this). Then record the cd onto your cassette through your cd/tape deck(usualy they have a one-touch recording option for recording a cassette from cd). Of course, you have to get a cassette that has the appropriate length, I usualy get the 90 minute ones.
Now for your question. Yes, you can record by connecting your sound cards line-out to your tape decks line-in. But if you choose to do it this way, make sure your soundcard's output is not amplified(you'll know this by using a headphone and increasing the volume slider.....you should be able to increase the slider all the way without blasting your headphone), if it is amplified then try to keep the volume to an appropriate level. You can set the volume to an appropriate level for recording by first trying out the soundcard's line out on a headphone.....just adjust the volume such that you here the music clearly but not too loud. I think the easiest way to record through this method will be to record your cassete one side a time. Say you have a 90 minutes cassette, compile the songs you want on your pc's mp3 player such that it is roughly around 45 minutes but not exceeding that. Then press the record button on your tape deck(make sure that you can hear the songs you are playing on the pc are also being played on the tape deck's speakers), wait for a few seconds then play the mp3 compilation on the PC witt the shuffle and repeat options disabled. Onece done, switch the cassette side and repeat the whole process.