You need a line out from your amp to go to a line on the sound card. You may not actually damage your sound card (but then again it may depending on the amp) however it will sound less than stella.
It won't hurt your amp, nor your computer. Wether it'll sound good is a different question, mostly because you are bypassing the driver which has a big effect on your tone. Basically, clean tones will sound a 1000 times better than distorted tones, but you'll find that out by just trying it.