I’ve done tonnes of stuff with 6 heads off one computer but that was 6x1024x768 no HD. Can suffer a bit if you try using heavy shaders and high vertex counts. You just need a really decent graphics card and very fast core speed on your processor.
I did a job recently with 4 x 720p outputs from 1 machine and even using some quite heavy content and it handled it well. That was with i7 core running at 4GHz and a GTX 580 3GB which kick ass BTW. Another job, same machine, 5 x 800 x 600 and around 10 render passes, so that’s around 24 million pixels being calculated every frame and still achieved 45fps!
All depends on content. If you’re doing lots of render passes you’ll def need tonnes of GPU memory and high clock speeds. CPU high frequency’s are important at high res’s to reduce CPU bottle necking. 4Ghz if you can.
Consider dropping down from 1080p to 720p if performance an issue, depends on install but most people won’t notice the difference. Also, high anti-aliasing will kill FPS at super high res, use 2 for early passes and 4 for late passes. Again, play with and see if you notice the difference when install live.
Also, develop content patches receiving a master back-buffer size coming from a S node and plumb into the renderer nodes. You can then tweak buffer size to help with performance. I then put a back-buffer multiplier on a S node and multiply this with some of my back-buffers in some render passes of my content. Often lower back-buffers in early passes really aren’t noticeable in final output, eps if using blurs. For instance set the multiplier to 0.7 and watch your framerate increase with out loss of quality.
How far the viewers are from the screen makes a big difference to how much they’ll notice too. There’s this whole thing about relative resolutions depending on viewers distance. Sometimes you can take advantage of this.
For me, it’s all about being about to tweak it quickly and easily for performance increases on site.