nVidia sells their GPUs to their partners, e.g., EVGA, Galaxy etc. Those companies decide how they want to market the graphic cards, however, nVidia provides them with a reference design which is what a standard say GTX 480 would look like. Apart from graphic card variants, there isn't much difference between them, apart from the warranty and customer service that they provide.
Palit, EVGA, Galaxy, XFX, ASUS...etc...those are manufacturers for NVIDIA or ATI which they only produce the PCB board and the cooling unit. NVIDIA & ATI do not make video card unless its for review or samples.
It is easier for NVIDIA and ATI to just make the GPU and sell it to the manufacturers to produce the card. That way they don't need to worry about backend(customer service, tech support) but to focus on marketing.
Actually in most cases a third party manufacturer (Foxconn for example) builds the cards based on nVidia's reference design (PCB, component selection for power and memory, and cooling solution). These are then packaged up and sent to nVidia's or ATI's AIB partners (AIB=Add In Board) where they place their stickers on them (branding and serial numbers) and put them in there branded boxes with there own selection of accessories (cables, adapters, case stickers etc) and these are shipped to retailers / e-tailers. These cards are essentially exactly the same.
This isn't the case for specialty (read high priced) cards that AIB's may produce. Some are simply reference cards that have been binned/qualified to run at higher clock speeds. Some are complete redesigns with cherry picked GPU's and faster memory. The latter cards usually include beefed up cooling solutions as well. The price is dictated by how far the card deviates from the reference, with the deviation paying dividends in performance. Usually these dividends are a case of diminishing returns, meaning the user gets less and less performance increases for his money.
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