Thought others might benefit from the solution to my problem, stumbled across after many hours of trying different things.

I upgraded from an X1800XT to an 8800GT and found I could no longer use any widescreen resolution below 1680x1050. I stumbled across this solution on another forum and have successfully added 1280x800 and 1440x900:

Solved my problem. I found this solution on 8800 GTX dosnt support 1440x900 ? - Tekbunker Forums and have successfully added 1280x800 and 1440x900.
1. NVIDIA Control Panel > Display > Manage custom timings.
2. Click both "Treat as HDTV" and "Allow modes not exposed by the display"
3. Click [Create...]
4. Under Custom display mode values:
Horizontal desktop pixels: 1440
Vertical desktop lines: 900
GDI Refresh rate: 60
Bits per pixel: 32
(don't have "Interlaced" checked)
5. Click [Advanced >>]
6. Under "Back-end parameters":
Timing standard: DMT
Desired refresh rate: 60
(the rest of the fields are grayed out)
7. Under "Front-end paramers"
Scaling type: Display (none)
8. Click [Test]
9. Click [OK]
Credit to ECPowers on Tekbunker forums

Edit: I should mention that using automatic setting while creating custom resolutions does not work, hence the need for this solution.