When it comes to GPU's yeah... you can mix then up, but some companies use custom PCB and clock rates which can effect performance. with ATI, I've always Stuck to Sapphire... and for a good reason, with two 6970 Im getting 56~59c Max load with the fan pushed up to 90% with no additional cooling, the same applies to my old 5770 which has lower temps. It might be to my case arrangements, but who knows.