Its' not really that good to get a GC USB adapter. It is nice, but there just aren't enough buttons and the digital pad is way to small. However for racing games, the two joysticks are better than the PS2 one as far as accuracy <since they have 8 cuts in each rather than a complete circle on the ps2 ones if you get what i mean>. And the trigger buttons are a hassle to press as a button, since you have to press them all the way until it clicks. But for racing games it is great.
I recommend getting a PS2 one instead. Same price, but i think it is an overall better controller for PC games, plus it has more buttons.
I got both from radioshack and they work good, except it doesn't support the wavebird

But the ps2 one supports wireless controller <the hip gear one i tried>.