Missing Video Modes

[Enverex]

XRandR is missing a few critical modes for playing old games, such as 800x600, I assume this is because my TV/Amp's EDID is being more than a little sparse when it comes to supported screenmodes. XRandR currently shows this:

htpc@topaz ~ $ xrandr
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 16384 x 16384
DVI-I-0 disconnected (normal left inverted right x axis y axis)
DVI-I-1 disconnected (normal left inverted right x axis y axis)
DP-0 disconnected (normal left inverted right x axis y axis)
HDMI-0 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 160mm x 90mm
   1280x720       60.0 +   59.9     50.0  
   1920x1080      59.9     50.0     24.0     30.0     25.0  
   720x576        50.0  
   720x480        59.9  
   640x480        59.9  
   480x576        50.0  
   480x480        59.9  
DP-1 disconnected (normal left inverted right x axis y axis)
  1920x1080 (0x27f)  148.5MHz
        h: width  1920 start 2008 end 2052 total 2200 skew    0 clock   67.5KHz
        v: height 1080 start 1084 end 1089 total 1125           clock   60.0Hz

Now I've seen it mentioned that you can add modes manually by doing this:

htpc@topaz ~ $ gtf 800 600 60

  # 800x600 @ 60.00 Hz (GTF) hsync: 37.32 kHz; pclk: 38.22 MHz
  Modeline "800x600_60.00"  38.22  800 832 912 1024  600 601 604 622  -HSync +Vsync

htpc@topaz ~ $ xrandr --newmode "800x600_60.00"  38.22  800 832 912 1024  600 601 604 622  -HSync +Vsync
htpc@topaz ~ $ xrandr --addmode HDMI-0 "800x600_60.00"

But that fails with:

htpc@topaz ~ $ xrandr --addmode HDMI-0 "800x600_60.00"
X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  153 (RANDR)
  Minor opcode of failed request:  18 (RRAddOutputMode)
  Serial number of failed request:  31

Despite the mode being listed now under xrandr:

  800x600_60.00 (0x2bc)   38.2MHz
        h: width   800 start  832 end  912 total 1024 skew    0 clock   37.3KHz
        v: height  600 start  601 end  604 total  622           clock   60.0Hz

Any ideas what I'm missing or any alternate ways of doing this? (or maybe getting EDID to list more than a few not particularly useful video modes)

Driver and card in case it's relevant:

OpenGL vendor string: NVIDIA Corporation
OpenGL renderer string: GeForce GT 430/PCIe/SSE2
OpenGL version string: 4.2.0 NVIDIA 302.17
OpenGL shading language version string: 4.20 NVIDIA via Cg compiler

I saw mention of doing this via metamodes, but it's had no effect from what I can tell (xrandr still doesn't list the modes and games that need them still crash):

Option "MetaModes" "1920x1080 { ViewPortIn=1024x768 } 1920x1080 { ViewPortIn=800x600 }"

Or rather it just resized the screen, I didn't get a new mode available in xrandr, which means that apps that need to change to this mode still think it's unavailable.

I need 800x600, any ideas how I can get it back? Also 1024x768 realistically (I know the TV/Amp support it).