Side-by-side monitor problem "2048x2048 max"

Question

I'm using Ubuntu 14.04 LTS, trying to configure side-by-side monitors

SystemSettings>Displays: I move the 2nd monitor to the right side

The selected configuration for displays could not be applied

Requested size (2880, 1024) exceeds 3D hardware limit (2048, 2048). You must either rearrange the displays so that they fit within a (2048, 2048) square.

(underneath works, but I'd like to fix it to side-by-side. PS: this hardware works in Windows 7)

lspci | grep VGA: (is this telling me I am using the "right" Radeon driver? (I do have a Radeon X600 video card))

01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] RV370 [Radeon X600/X600 SE]

xrandr: (is this telling me I can use up to 4096x4096?)

    Screen 0: minimum 320 x 200, current 1600 x 1924, maximum 4096 x 4096
    DVI-0 connected primary 1280x1024+320+900 (normal left inverted right x axis y axis) 338mm x 270mm
       1280x1024      60.0*+   75.0  
       1152x864       75.0  
       1024x768       75.1     60.0  
       800x600        75.0     60.3  
       640x480        75.0     60.0  
       720x400        70.1  
    DVI-1 connected 1600x900+0+0 (normal left inverted right x axis y axis) 432mm x 240mm
       1600x900       60.0*+
       1280x1024      75.0     60.0  
       1280x720       60.0  
       1024x768       75.1     70.1     60.0  
       832x624        74.6  
       800x600        72.2     75.0     60.3     56.2  
       640x480        75.0     72.8     66.7     60.0  
       720x400        70.1  

Earlier, Fabby asked about my GPU memory, is this what info is needed?

lspci -vnn | grep VGA -A 12
01:00.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc. [AMD/ATI] RV370 [Radeon X600/X600 SE] [1002:5b62] (prog-if 00 [VGA controller])
    Subsystem: Advanced Micro Devices, Inc. [AMD/ATI] Device [1002:0b02]
    Flags: bus master, fast devsel, latency 0, IRQ 45
    Memory at d0000000 (64-bit, prefetchable) [size=256M]
    Memory at fe9e0000 (64-bit, non-prefetchable) [size=64K]
    I/O ports at dc00 [size=256]
    Expansion ROM at fea00000 [disabled] [size=128K]
    Capabilities: <access denied>
    Kernel driver in use: radeon

01:00.1 Display controller [0380]: Advanced Micro Devices, Inc. [AMD/ATI] RV380 [Radeon X300/X550/X1050 Series] (Secondary) [1002:5b72]
    Subsystem: Advanced Micro Devices, Inc. [AMD/ATI] Device [1002:0b03]
    Flags: bus master, fast devsel, latency 0