New video card not detected

Hello , i just bought myself a jewel for my gaming experience but when i plugged it in my pci slot it doesn't start. The card is GF GTX 460 INNO3D 1GB 256-bit. I unplugged my old card ( ati radeon hd 3300 ) went to the shop and came back with the new one. I plugged it and i started the pc. Nothing happened. Then i switched the monitor into the mainboard and when the monitor flashed, it was loading windows 7. When windows loaded it was recognizing the old video card because it's driver was still there. It didn't detect that i have plugged any new hardware. When i insert and run the driver DVD included with my new card it also says that i don't have device compitable with the drivers it wants to install. Truly i am with my generic mainboard video at the moment and at least it allows me to start my pc and try to solve the problem. Is it that i don't have enough powerful supply? Is it that the video card isn't compitable with my mainboard? Is it that i haven't plugged it right, it surely goes smooth into the slot , but the card has 2 ( my old card had 1 ) power supply hubs to stick it in. Tried both didn't work.

My power supply is Flotron 400W
My processor and mainboard are:
AMD Phenom 9650 Quad-Core 2.31 GHz
4 GB RAM
Motherboard ASUS M3A78-T AM2+
OS: Win7 Ultimate

On the official website it says this :

• 1. A PCI-Express compliant motherboard with full size expansion slot>
• 2. 2x 6-pin PCI Express supplementary power connector

i have plugged only 1 , will try how to plug the 2nd one too and see if it works

Yip, the GTX 460 INNO3D requires both PCI-e 6pin power leads attached. 400W PSU is a bit low. A quality 450W is the lowest recommended.
You may also need to go into the motherboards BIOS and change the VGA detection order, so that it looks for a graphics card on the pci-e slot instead of using the onboard HD3300.

Plus, did you uninstall the ATI card (from Remove Programs or from Device Manager) before you shut down to install the new nVidia card?

You can't just switch cards without preparing the OS for the switch.

The problem was that as you said it required both 6pin power leads in. The box of the videocard contrained only 1. shame on them. I went and bought for 10 euro 3 more because only 1 goes our of my power supply. It's all connected now and working. The video card is pretty powerful and so far i don't experience problems with not having the 450 min W requirement. I hope it stays like that. Thanks for info guys.