It'll work fine. Was a problem under Vista initially, now back to goodness. I'm running a mixed-GPU setup similar to what you're wanting, at work. I have the HD 2000 driving my main monitor via HDMI, and a GeForce 8600GT driving two more. You might have some issues with the GPU not being fully enabled/functional without a monitor hooked to it. You can make a dummy VGA plug or just drive a monitor with it, to get it fully functional. You'd basically be doing Lucid Hydra in reverse.
Windows 7 does do some bizarre fruity stuff with multi-mon on multi-GPU. I don't fully know what's going on under the hood, but it looks like Win7 does some nifty framebuffer-copying nonsense, so that I can run primary monitor off a GTX 460, run monitors 2 and 3 off my 8600GT, and run a DX11 app on any monitor. It's kind of useful, but there's a performance hit across the board.
Anyways, for what you want, should be fine, worst case you grab a cheap LCD to hook up to your GPGPU card, to get it to full speed.
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