Nvidia 6600 gt vs 8400 gs

[yo11yo]

I have two computers at home one is twice as better then the other one but
I don't know what graphic card is better the 6600GT or the 8400GS. I use my computer mostly for gaming.
Can I use both Graphic cards one the same computer or its better not?

Thanks.

 

[Mousemonkey]

I've had both and the 6600GT is indeed the better gaming card.