To bump an old thread: I solved my triple monitor setup. Spectacularly cheaply too by taking advantage of some second hand kit.
I added a second graphics card alongside my NVidia 6600. The cheapest DVI card I could find was an ATI Radeon 7000 PCI. Yes, I took a risk in getting a non-NVidia card.
I setup both cards in xorg.conf and they worked together. I encountered some bugs, lockups etc. (especially when trying to combine all 3 screens into one desktop using Xinerama). I tried many different sets of X packages with mixed results, and eventually found if I forcefully loaded the kernel 'radeon' module on bootup and these all went away. X doesn't load this module automatically. I'm using the most recent X packages I can find now.
This left one remaining problem: OpenGL caused the server to crash instantly. I traced this to a bug caused by having the environment variable __GL_SYNC_TO_VBLANK set, when having both my NVidia and ATI card on the same desktop. Disabled this, and I get OpenGL using the custom libGL on the NVidia card, and black on the ATI card -- perfect. I've sent in a bug report on the NVidia drivers here.
The setup works with or without TwinView driving the two NVidia displays. I'm finding not using TwinView to be better, as then I don't need 'fake Xinerama'.
So, if it's useful to anyone, here's my setup:
* Slackware on x86
* xorg-server-1.6.2 and associated packages, including xf86-video-ati-6.12.2 (all from Slackware-current)
* NVidia drivers 185.18.14
* NVidia 6600 PCI-E with two DVI outputs, connected to 20" 1600x1200 and 24" 1920x1200 TFTs
* ATI Radeon 7000 PCI (£12 from eBay) with DVI output connected to 20" 1600x1200 Dell 2007FP (£102 from eBay)
I've been running for a day now, all seems okay including OpenGL (on NVidia only). Not bad for a total of £114 spent either. Now I just need to get back to work
I'm sure I'll discover some niggles in the coming days too. But right now, I'm very happy!