some basic things to try first...
1) Try different refresh rates
2) Check your computer temps, heat can cause a few issues.
3) In your BIOS, try different AGP aperture settings. Most seem to agree on the "128" setting.
4) Also in the BIOS, try disabling Fast Writes.
5) Try a complete uninstall/reinstall of the drivers. Theres a few threads on how to do a complete uninstall of previous drivers without a reformat. I would recommend this especially after changing from ATI to Nvidia or vice versa.
6) Have you tried any different driver revisions? http://www.guru3d.com/
has plenty of the newer beta drivers.
Personally I use the 61.77s myself and have no visual troubles, but I reformatted as my last card was an ATI 9800XT.