Video card wont fit in my dell OptiPlex gx280

SOURCE: Dell Optiplex GX280 boot problem

I just repaired two Gateway systems with similar behavior (turn on power, fan runs at full speed, no boot), and both needed replacement system boards. (These were a different model, but the symptoms are typical of a dead system board.) I got the boards from a company called Blue Raven Technology (www.blueraven.com).

Before deciding the main board is bad, try another power supply to make sure that's not the problem. Also you might try clearing the CMOS memory on the system board (either by moving a jumper usually found near the backup battery, or by removing the battery itself). Sometimes corrupted CMOS data will keep the system from booting.

SOURCE: Dell Optiplex GX280 Video Card Install Problem

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

SOURCE: dell optiplex gx280 makes loud fan sound at

You may have a problem with your power supply(the loud fan noise) and eventually it may get too hot and will cause problems like not fully booting.

SOURCE: No Display in dell optiplex gx280 & Fan speed is veyy high

GM from pakistan fsd problem in motherboard chepset......you should repare u r gx 280 or heat the chepset with heat gun ........ 

SOURCE: problem withe dell gx280 fan and display

I had exactly this problem on an Optiplex GX 280.
I replaced 1 stick of ram, and everything was happy.