I have a Visiontek 7850HD clocked at 1000mhz. If I want to run in Crossfire mode with another 7850HD do the core clocks have to match?
Yes and no. It's better to have another GPU that is identical - mainly for aesthetics and clock speeds to be the same, but it doesn't need to be identical. If you choose to purchase another GPU that doesn't match your current VisionTek Radeon HD 7850 with 1GHz on the core clock speed.
If it doesn't match it, both GPUs will slow down to the slowest one. So, if you buy another HD 7850, and its core clock is say 950MHz, your original GPU at 1GHz will slow down to 950MHz.
I would suggest buying another VisionTek GPU, but if you can't - you should just be able to overclock the new GPU up to 1GHz to match your current GPU. Happy CrossFiring!