Thanks both of you for the quick replies...
This is the motherboard they will go in: http://www.asus.com/Motherboard/M4A89GTD_PROUSB3/#overview
Same clock? Well, that's just the point. The one I have now has a 870mhz core, the one I wanted to pick up has a 810mhz core. Slightly different memory clock rates too. (see above links) I thought I read where the faster card will drop its clocks to the speed of the slower card. No?