Right click an open area of the desktop then go down the list to Nvidia Control Panel. Left click it. Just in case Click the pic and it gets bigger.
This takes you to Nvidia Change Resolution page. See pic below.
1)Select monitor- your new one should be outlined all ready.
2) Select PC resolution. As you see mine is 1440x900 at 75MHz. Almost all monitors use setting at 60MHz so yours should be 60 MAYBE 75MHz.
If your resolution is in this list you are done. Close page. Only go to Step 3 if yours isn't listed at Step 2. Custom Resolutions as the ones I have shown are totally unstable with MY monitor. So don't go further unless necessary.
3) If your Resolution isn't listed click "Customize"
4) Select a PC res that may work. You can always change or try others.
5) Click OKAY to accept and try.