ATI doesnt make its own cards, neither does nVidia, they both release a reference card to OEMs like Gigabyte, ASUS, XFX, and others, the early version of each card is always the same as the reference cards, after a while the OEMs make custom boards and custom coolers to make their card better in some way than the reference card.
if you are finding an original ATI HD 4550 that was made entirely by ATI its either an engineering sample or a fake.
that not true at all, custom card are tend to be cost down version compare to original reference in most of case. especially mid range and lower mainstream model are full of crap if they were made by oem's custom design. however since it's an "4550" i wouldn't recommend to go for gigabyte unless they're still have the reference.