What is minimum Amp on +12 V rail required to run ATI Radeon HD4550

i can't because my pci-e 16x 2.0 is only 25W max no 75W

Yes i know but my pc is a dell optiplex gx620 smalll form factory and i have seen the documentation and there is only 25W from PCI-E 16x 2.0

The gx620 has the option to come with a x600 which uses ~40w. I do see 25w on the spec sheet but the 75w is a mobo implementation after 1.0 and you have 1.0a. You never said why you need to upgrade but at this low end I wouldn't expect gaming unless it's old games. The 5450 uses less power than the 4550 and is better performance. I would say that a 6570 should work but depending on your uses, you may not even need it.

Edit: Nvm I read your other thread and do see you are trying to game. If you can't afford a new pc I'd say go with a 6570 which can be found used for ~$45. If it doesn't work sell it on craigslist of somewhere else for the same price. Googling shows plenty of +25w cards working on the gx620.