I have a older pc my nephew uses that does the same thing. Onboard is a NV6150and when I put an Nvidia vga in it boots just fine to the add in NV card without disabling the onboard 6150. If I put in a ATI card it will work just once then the next reboot it goes back to the NV6150 and I have to disable the onboard 6150.
Since your onboard is ATI 3300 and has no problems with a ATI add in VGA without disableing the onboard you have to disable the ATI onboard. I assume you are useing the correct PCIE slot. The manual for your MB says you have to use PCIE2 (Green)
From the manual page 21
If you plan to install only one PCI Express VGA card on this
motherboard, please install it on PCIE2 slot (Green).
There is a setting in your Bios that will change the PCI Bus scanning order while searching for video card. It is under chipset configuration and is called "Primary Graphics Adapter" Default is PCI, change it to PCI Express and see if that works.
From the manual page 62
Primary Graphics Adapter
This item will switch the PCI Bus scanning order while searching for video
card. It allows you to select the type of Primary VGA in case of multiple video
controllers. The default value of this feature is [PCI]. Configuration options:
[PCI], [Onboard] and [PCI Express].
Like others have said this isnt a bad card. Good luck.