Manufacturers come up with ATI Radeon HD 5750 & XFX has come up with HD 5750..... Is there anything like ATI HD 5750 is original and the other one is just the replica of it with lesser price ???????
No no, not at all. Basically, AMD or Nvidia come up with the design for a card, from there manufacturers like XFX or Asus take the reference cards and adjust them or make improvements on them. Sometimes, they don't really change them at all!
For many of the cards that different makes come out with, the only difference is the sticker. Sometime they create a new cooler for it or offer pre-overclocked versions, but the card generally stays the same.
The actual cards are manufactured by a few OEMs (original equipment manufacturer) who basically slaps on the sticker of the various brand names out there. Two such ATI card manufacturers are PC Partners (the parent company of Sapphire) and Pine Tech (the parent company of XFX).
ATI/Nvidia make the GPU's themselves, I think the various vendors(brands as you called them) just change the types of memory on the card, type of cooler, and video out puts. I may be partially wrong, but I think that's how it works.