I'm planning on picking up an eVGA Geforce 7900GT. I'm getting an SLI compatible motherboard, and later down the road, I might get another 7900GT to run them in SLI. I know that both cards have to have the same GPU, but does it matter if I mix and match the brand and clock speed? I just fear I won't be able to find the exact video card in the future when I decide to go to SLI.