PC is not booting Dell Optiplex 100

Are you using a four pin connector( connected to power supply)? if yes it will give you error. Connect the fan as shown in the image

you seeting your pc's battery option or checked the connection.

if you have two partitions on your hard disk drive that means that D partition is dead! If you have two separeted hard disk drives C and D check the flat and power cables, check the jumpers on hard disks, switch the possitions of the jumpers on drive D to SLAVE position!

Bummer, but what you describe is ususally some cable dislodged. Remove and reaseat everything and try again. Remove and reseat the ram, leave the processor alone.

The blinking Amber power LED indicates that the power supply has a failure.

checked replacing ram and the motherboard that should fix ur problem or else try booting ur system without modem card plugged it and also by pass surge protector .

You hit that one right on the dot.  ATI website showed the specs for that graphics card you installed as needing a 300 Watt power supply.  Good news though... I researched newegg.com and I believe I found your 300W power supply.  There are 4 different Optiplex gx280 designs with 4 completely different power supplies.  Since you bought the low-profile video card, I believe you have the slim computer case and this power supply should work for you...
http://www.newegg.com/Product/Product.aspx?Item=N82E16817374002

Good Luck

ok
open cpu cover and refix RAM and connection of power cable and data cable to harddisk.

check whether ur ram is working or not. remove the cpu side door and remove the ram from the ram slot and clean it using a eraser gently and fix again it back and power ur pc on and check it

its going to rather your power supply or your motherboard has died, id try a different power supply first