To the casual observer, there is little difference between the AMD Radeon HD 7970 and AMD Radeon HD 7950 refernce cards. As we have highlighted earlier, AMD Radeon HD 7950 features the same Tahiti GPU, but with a slightly lower number of 28 GCN CU versus the 7970's 32 GCN CU units while the number of TMU units also differ at 112 and 128 respectively. Additionally, memory clock speed has been clocked down to 5000MHz instead of 5500MHz on the top model. Such seemingly miniscule differences are typical of SKU differentiation for graphic cards.
In terms of size, both cards occupy two full expansion slots and are as long as their Radeon HD 6900 predecessors at 10.5 inches long or about 27cm.
Big brother Radeon HD 7970 is rated at 250W for its maximum power draw of its circuit board while the Radeon HD 7950 is rated at 200W. That said, for a simple and straightforward gaming rig, you probably won't require anything more than a well-built 550W PSU to get these GPUs going.