Is the EVGA an 'eco' version? I thought the minimum core clock on all 8800GT/9800GTs was 600mhz. Perhaps your one is downclocking?
Download EVGA Precision and you should be able to set them both so they run at the same clocks, you may have to choose some middle ground if the lower clocked one can't keep up with the Gigabyte clocks, but you will end up with better performance as they usually both end up running at whatever the slowest card's clocks are. I run my 8800GTs at 675/950 for everyday.