I'm not too confident that it would help fix the problem, but its certainly worth a try, the only downside is that you'll lose all your PRAM settings (sound volume, alert sound, date/time, network settings, etc). The CUDA reset switch is a tiny black or red switch somewhere on the top side of the logic board.

Unfortunately though, I'm willing to bet that there's a good chance that there's a problem with your card - having done it myself I can confirm that adding a USB card to a Mac, even an older Mac running OS 8.6 should not be this hard.