Well onboard sound really has improved in the last 5 years. If your using a Creative Labs board - AKA sounblaster - check here to see if your can use it with Win 7.
http://support.creative.com/kb/showarticle.aspx?sid=61105
Onboard will be supported out of the box without question and usually is good enough for the average ganer, but each person has their own important things, so if you feel you get better quality out of a card, go for it.
I do have to say that both SH3 and SH4 had problems with audio "tearing" on my rig with XP. I kept my SB Live card forever only to use the gameport, then finally went USP for the joystick, and out the card came. Still, that is a pretty old card, so no big suprise. I can't recommend if one is worth it or not because I don't use one anymore. If I did alot of audio stuff I probably would want one. Just not my thing. Maybe one of the other folks on here with more knowledge can fill you in on the pro's and cons a little more.