If the problem is occuring with the onboard ethernet I wouldn't count on it being reliable in the future. Perhaps when you powered down the computer to install the PCI-e card that may have been enough to gltich the onboard ethernet circuitry to start working again, but I would bet money that you will encounter the same disconnection issues down the road. I would certainly see if it continues to work and just have the PCI-e card as a backup that you can switch to quickly. You never know hey?
I believe the Vostro 460 has PCI-e slots which don't work with older PCI cards so yes I think only PCI-e ethernet adapter cards can be used.