It doesn't matter if you have drivers or not - you should have a picture on your monitor.
Take the new video card out. Put your monitor cable to the onboard VGA socket. Do you have a signal?
If you do, go to bios and there will probably be an option which graphics adapter is primary (onboard or PCI-e). Select PCI-e, save and shut down the computer and reinstall the video card.
If you don't have a signal from the onboard vga (or from the video card), we need to do some troubleshooting... Just my two bucks, but I'd go something like this:
1) Try different VGA cable or monitor. Or check the cable and the monitor in a different computer.
2) Do you hear any beep error codes when you boot up the computer and judging by the power and HDD lights, does the computer start to load the operating system? Do check that you do have a beeper installed in the system.
3) Check the motherboard for bad caps (bulging etc.) Wikipedia - Capacitor plague
4) If you can, try to boot up with another power supply. Especially if it seems that OS does not start to load but fans etc. are whirring.
5) Remove all extension cards, HDDs, DVD-drives etc.
6) Reset BIOS via the onboard jumper
7) Remove onboard memory and try to start.
8) If no beep errors -> remove motherboard and cpu from the housing; if you do have beeping -> Possible cpu failure, try with another cpu. (RARE)
9) With only motherboard, cpu and power supply installed on a "tabletop" configuration (please put esd plastic bag, and/or cardboard or something under the motherboard to avoid shorting), try to start.
If it starts okay, your housing is bent, and it shorts out the motherboard. Probably though is doesn't start and you have a dead motherboard.