The 1Ghz one will downclock itself, so it is a waste of overclock, I suggest you to overclock your 925Mhz one to 1Ghz also. But Either way it is suppose to be OK on crossfire .
With MSI you can bring up the other card to match or you can turn down the OC'd card to match. That is the beauty of MSI Afterburner. If you do nothing the OC'd card will auto run as the non OC'd card that is what it is supposed to be but other people say different and I don't 100% know but I do know you can crossfire them and then adjust them
I can reduce or increase frequency in catalyst control panel, why use MsI afterburner for that? And why specially MsI? There are Asus and gigabyte tweakers for gpus as well...
afterburner is just what people like using since it is pretty decent.
CCC should be good enough for what you need it for.
Ya I keep forgetting about the ccc on the AMD side since I use Nvidia.
MSI Afterburner is what you should use, everybody use it as their software of GPU tweaking...