Well it's simple really. I run two displays on my main system. On the primary display I play my games. And on the secondary display I run TeamSpeak, browse the web, and play Winamp. Currently I have both displays hooked up to an 8800GTS/640. And I've found that games suffer very slightly because the one card is also handling things for the second display. Technically, what I want is to buy two GTs and move the GTS/640 to my secondary system. An important reason is that Nvidia's drivers have one feature that ATI's drivers have always lacked. They can scale output vertically without stretching it horizontally. This is a feature that is required for using Widescreen displays with many older games.
When I heard (in this review) that ATI was going to make Dual Display work with CrossFire, it piqued my interest. But then it didn't take long to realize that I couldn't count on this to work, just because some marketing type said it would!! In fact, I have grave doubts that ATI's drivers will be as good as they were before. Simply put, I suspect that AMD will be so strapped for funds in the near future that they'll cut development for Everything. And like I said in the past, this "launch" seems to be a clear indication that they will never again be anything but second rate. That applies to both their hardware and drivers.
Please note!: I don't want to see AMD / ATI go belly up! But they simply can't compete on anything but price (or as some might say "value"). Until things change fairly drastically, I think I'll opt for a pair of GTs and there isn't any reason why a 256MB version shouldn't be able to handle Aero on that second display.
Wait, so you'd but a second 8800GT just to run the second monitor with TS, Firefox and WinAMP? What's the logic in that-- Aero can use any other nVidia card that uses the same WDDM driver for a second display, and if you're not using it for gaming, why not just an FX5700 or 6600LE card? If you're thinking of SLI for the 8800s, you're stuck-- SLI disables the second monitor, and breaks the ability to add a third PCIe or PCI nVidia card, while Vista turns off Aero and falls back to old drivers if you're not using the same WDDM driver. This is the exact fight I am in; I bought a pair of 7900GS cards and a PCI FX5500 before I knew of the limitation with SLI... I put in a Matrox P650 for my second monitor, and now can't get them both to work on Vista.