I'm not a hardware specialist, so every time I want to buy something, I do a quick research to see what's currently available in stores, what's already a standard and what's an expensive novelty. Next I imagine in my mind a power(price) function and find its inflection point to see what product is most reasonable to buy ;)
I've been fan of Intel and NVIDIA, but I must admit that now my PC is dominated by AMD. Some time ago I wanted to buy a 4-core CPU. It turned out that Intel i7 processors are very expensive and that's how I've bought AMD Phenom II.
Now it's the same with graphics cards. I thought it's a good time to buy one with DirectX 11 support. I've been waiting for some time for these new cards from NVIDIA 4xx series, but now as they are available in stores I see they are very expensive, while I can have a GPU with DX11 support from AMD for about 600 PLN. My choice is Radeon HD 5770. It is not the most powerful card available (it actually has its power cut by half comparing to its higher versions), so I can see it's the speed what costs most. While for me the capabilities are more improtant. I just wanted a card with DX11 support. Of course AMD cards don't have CUDA technology, but well, it's vendor specific anyway while DX11 has Compute Shaders.
BTW, anyone wants to buy GeForce 8800 GT? :)