Well, I wasn't really using my brain too well. The reason the one card is getting around 640kh/s is because it is the card used by the PC for graphics. The other two are dedicated miners. Now, for the first card I had, the original one that was getting 540kh/s, that still seems low, even though it was my only GPU on my desktop. This weekend I plan on mounting all the guts onto a platform and running all 4 of them, see how it performs when it's dedicated.