Processing cores refererse to how many cores they can have... The more the better... yes, it is that simple. That is why the special editions of the 260 with extra stream processors are more expensive.
Each of these is a single gpu. You would need an SLI MoBo to go duel with either. BTW, these could also go tripple SLI on the right MoBo.
I wouldnt consider the 280, myself. It is faster then the 260 but not enough to justify half the price difference. The 260 is likely to take you at least another 18 months before it even starts to look outdated.
The 260 and 280 are single GPU cards. However, each GPU has multiple pathways or stream processors which allow parallel processing. In short, a single GPU can process many tasks at the same time instead of waiting for a prior task to finish before starting or queueing another task.
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