Windows 7 problem with ATI radeon HD 4770

Hello everybody:
I recently bought a xfx Radeon HD 4770 for use on my Windows 7 O.S. Installed the card on the MoBo, and changed display priority from OnBoard to PCIe in the BIOS. Then, during boot sequence a flashing blue screen appears. I cannot boot into Windows 7 o.s.
I tried the different boot (start) options at the boot screen, but all of them (even the Repair option) takes me to the Blue Screen with code 0x000000A5, telling me about the BIOS not being fully ACPI compliant. I don't understand this, since this BIOS have the ACPI option - I have been using Windows 7 o.s. for a while, with the onboard VGA.
Help is much apreciated.

Read more here - http://support.microsoft.com/kb/831691

Please note:
Even though it was not memory you added. When adding the graphics card to your system, you are freeing up memory on your motherboard because the graphics card has its own memory.

One question I have after reading this.
Did you shut down your computer?
Or did you place it in Hibernation?

If you are in a hibernation mode. You can always remove the card, boot back up, and then shut down properly to add the card.

Thank you for the replies. You answer me very quickly, I see.
No, I never place my PC in hibernation mode (sorry for the english).
(i.e. I never hibernate the PC).
I shut down the PC in order to place the graphic card on the Motherboard.
Then, I cannot boot the Windows 7 since I installed the new card.
Even that, I didn't know I could install a new hardware while the PC's hibernating. I never did, indeed.

Have you checked you computer manufacturer to see if there was a BIOS update available for download.

Which computer do you have?
Maybe we could look to see what we can find.

This may be silly question; did you go into the BIOS and switch the graphics selection from the on-board graphics to the PCI-E slot?

If not, that may be causing the issue.

P.S.
I saw where you did that... my brain fart.

Here, I stated "... and changed display priority from OnBoard to PCIe in the BIOS. " this in my first post.

The specs for my PC are in this page, left of this post.
There is no computer manufacturer - I assembled it myself. That's why I tagged it "custom"-assembled.
As a side note: the monitor has DVI input only. When using the onboard VGA, i used a VGA-DVI adapter. With the new card, as it has DVI output only, there is not use for the adapter. I'm not using it right now, because of this problem. I'm using my other, P-III PC.

So, I need to update the bios. But I will do that when the new CPU comes.
I wrote to XFX support, and they suggested me a clean install. I'm highly positive that this issue can be resolved without the 'harass' of a clean install. However I'm glad that you all pinpointed the possible 'causer' of the problem (again, SORRY for the english).

Now, I will thank everybody who helped me in this thread.
May the moderator close this thread, as I will post the results of the suggested action (updating BIOS) when (just before) I upgrade the CPU. It will not be in weeks, I assure you. Thanks everybody.