It could be a couple of things, though I doubt that you killed anything.
1. Try reinstalling your old card. If that works, then at least your system is working fine.
2. Does your system display anything on start up, such as the BIOS splash screen? If yes, then it's a driver issue most likely in Windows; if not, then it hardware.
2a. If drivers are an issue, boot into safemode by hitting F8 before the Windows splash screen bootloader. Uninstall the drivers, use a driver removal utility such as drivercleaner and reboot. This should allow Windows to boot without any issues (and any nVidia drivers). Download the latest ForceWare drivers and install them. This should work.
2b. If it is a hardware issue, make sure your power supply has enough amps on the 12V to run your shinny new video card. If you do, check the seating of the card. If that still doesn't work, try to see if the card works in another machine. If it does there, then it might be incompatible with your motherboard.
That should give you a start on what to look out for. Feel free to ask any other questions you have.