Trying to Install Radeon HD 5850 into Acer T180

Hi Everyone! I am trying to upgrade the graphics card for my daughter so that she can play Sims 3 without problems; however, I have been working on this thing for DAYS and still haven't gotten it to work! I am at my wit's end! I upgraded the power supply to 600W. I have seated and reseated the card several times. I have reseated the RAM. I have gone over all of the new connections to the power supply to be sure nothing was missed. I have tried to install drivers, uninstall drivers, change bios, etc. NOTHING WORKS! I cannot get the system to recognize the card! It has never gotten to finding and installing new hardware. If I leave the monitor in the card, I get no signal. If I plug in VGA, everything works find. The computer is running Vista 32 bit so I know it's not the U... thing. I am sure I have the video card in the correct PCIE 16x slot. There's only one...it's orange. I cannot find a newer BIOS version to flash. I am simply out of ideas. I'm now wondering if this card is even compatible with her motherboard.