For example, obviously it doesn't matter what type of adapters a video card offers when it comes to Bitcoin mining? Is the GPU speed or the amount of RAM the most important part in terms of megahashes per second?
The GPU speed when hashing SHA256 is the most important requirement. A good graphics card for mining this are usually ATI brand cards as these work by using a massive array of parallel shaders.
Nvidia cards generally perform with a higher floating point performance but achieve this via different means which dont perform as well against SHA256
Other hash types such as scrypt that Litecoin uses has some requirement for memory also, native clock speed is generally the defining factor on performance though.