Well, my 2cents:
Ati/OpenSource: Dual-Screen with xrandr works great on the open source ATI-drivers, I don't think TwinView could be much better. I also managed to play a mission in WoW (Icecrown) on my HD4870, but it still has glitches and crashes and you don't get eye-candy. So if you want to game seriously, the OpenSource driver is not there yet, especially if you go for Evergreen (no 3D yet, but work on that is visible).
Ati/fglrx: worked quite well for me, but on the other hand I didn't use it much. Biggest problem is the tremendously lagging support of new X servers, so if you want to live on the edge, fglrx doesn't work.
Nvidia/OpenSource: Compiz works, suspend works, gaming doesn't on NV50 (that should give support up to the GTX295 afaik). I regularily use nouveau on my laptop for quite a few months now and it is definitely usable and quite stable (well, the screensaver not so much ;-) ). GTX4xx support might take a while, as there is no support from Nvidia for this driver.
Nvidia/Proprietary: Worked great for me. If you don't care for OpenSource and want the most features and the best 3D acceleration, this is the way to go at the moment. Also has VDPAU. Actually I bought Nvidia cards for their linux support for years, until AMD started their OpenSource initiative.

Hm, a word on the GTX4xx: I wouldn't want one: All reports I read say they get really hot and they need a incredible amount of power. And I just think it's not worth that and the price premium for ~10% more performance. Additionally, with temperatures above 90C, I could very well imagine that those cards don't live too long. But maybe I'm sort of biased, as I already had a Nvidia card (7800 I think) that blew up just after a few months - and the replacement 7900 had artifacts and had to be replaced again - with an rather unstable card during summer (at least I believe it was the graphics card). Also the GTX4xx might have very serious supply shortages, so you might have problems finding one in the near future.

So, it's really a bit of a dilemma. My personal stance is that I'd go with ATI for their support of OpenSource - but that means you have to wait at least some more months ... and maybe years. ATI has cheaper, cooler and more power-efficient cards, but it doesn't have video-acceleration, support for recent developments (in fglrx) or top performance.
If you don't care for power, temperature, noise, price and closed source but want top performance in 3D and want it now - go with Nvidia.