1. First, if the only thing you replaced was the power supply unit, then go back and make sure you connected all the wiring from the PSU to the motherboard.. If there is a separate power cable required for the ATI card, make sure it's attached correctly.

2. Replace the CMOS battery as it appears to be failing or gone entirely.. It's a cheap fix and when the CMOS battery fails, it can cause the BIOS to default to its original settings when power is removed, causing the onboard video card to be default.. See the link below for instructions:

How To Replace The CMOS Battery In Your PC(Procedures and Pictures)

3. Next, make sure the ATI card is enabled in Device Manager.. The bad CMOS battery may have caused it to default back to the original onboard video card.

4. If that doesn't get the new card working, with the monitor cable plugged into the onboard video port, uninstall the ATI video card entirely, then reinstall it.. Uninstall the ATI video card driver from the Device Manager, shut down the computer, remove the card from the PCIe slot, then restart the computer.. Once that's done, then install the ATI card per the manufacturer's instructions, making sure to download and install the latest drivers for the card from the ATI website.

4. And if that doesn't get it done, then it sounds like both cards may be enabled in the Device Manager, allowing the default onboard card to take over.. This part can get a little dicey, but may be necessary...... So, with the monitor plugged into the onboard video card, start the computer.. Once there, make sure you've got the newest driver for the ATI card, for your operating system, is installed..

Next, access the Device Manager and click the + sign next to the "Display Adapters" section. I'll guess that you should see two separate video card devices.. Open the properties for the ATI video device and once again, make sure it's enabled at the bottom of the window.. Click on Apply, OK, if you make any changes.. Next, open the properties for the onboard video device and disable it. Once that's done, you'll be asked to restart the computer.. Instead of doing a normal restart, simply shut down the computer. Once the computer is shut down, unplug the video cable from the onboard video jack and plug it into the ATI card's port.. Now restart the computer.. It should recognize the new video device and allow things to work properly.

If it does not, then the ATI card may have been damaged, or the PCIe slot may have been damaged, so the card can't be recognized.. Swapping in a known "good" card would be our next step. You should be able to plug the monitor cable into the onboard card, restart the computer into Safe Mode which will load default VGA drivers, allowing you to re-enable the onboard card in Device Manager, so you can use the onboard card in "normal" Windows.

Hope this helps.