I ended up using HD 4000 as 2560x1600 with the displayport.
With Asus P8Z77-V, set the Graphics Memory to 64MB and iGPU as primary. MacPro 3,1. Works perfect so far.
<key>EthernetBuiltIn</key>
<string>Yes</string>
<key>Kernel</key>
<string>mach_kernel</string>
<key>Kernel Flags</key>
<string>darkwake=0</string>
<key>Timeout</key>
<string>0</string>
<key>UseKernelCache</key>
<string>No</string>
<key>Graphics Enabler</key>
<string>No</string>
<key>device-properties</key>
<string>7f00000001000000010000007300000002000000 02 010c00d041030a000000000101060000027fff04002c000000 4100410050004c002c00690067002d0070006c006100740066 006f0072006d002d0069006400000008000000050062011400 00006800640061002d0067006600780000000d0000006f6e62 6f6172642d31</string>
<key>Graphics Mode</key>
<string>1920x1080x32</string>
FYI - I used an adapter to get DVI. $45 on eBay.
DELL BIZLINK DISPLAY PORT to DVI DUAL LINK ADAPTER USB POWERED CN-0XT625