First off, always uninstall old drivers before installing a hardware upgrade. So you made the proper choice there.
Next, your power supply is most likely the problem. 200w is not nearly enough. I believe a 350-400w PSU is recommended for 7 series cards. I once upgraded to a FX5700 in a system with a 200w power supply and my PSU simply could not handle the load...
So get a better PSU. It's the unsung hero of a computer. Don't skimp on it. Check out the PSU guides on the site. Some great guidelines.
Once you get a new PSU, let us know if things work out for you.