Last I checked Compiz doesn't play well with dual monitors, so you may have to give up that particular requirement.
After that, any of the newer ATI Radeon HD cards should work well, although the very newest doesn't have specs released just yet.  They'll all have HD video out with Free driver support when the Free driver developers get around to implementing the hardware decode circuitry support, the latest (3800 series) cards have built-in hardware decode called "UVD".  Video out is not high priority right now, though, so Free driver support may take a little while.  Pick a card (prefer direct from AMD, IMO) with at least 1 port for video out (the AMD cards all have multiple) to make sure you can use that support when it becomes available.

Your board is a nForce 500 SLI MB, so it has 2 PCI-Express x16 slots and 2 x1 slots (between the x16 slots).  You should buy a PCI-x card, if you buy 2 SLI cards you can gang them together to act like one big dual-GPU card (some of the new cards, like the 3870, do this internally on one card), although I'm not sure if the drivers (binary or Free) support that yet.

==Joseph++

P.S.
Your browser thinks the page is a gif because it ends in .gif and the server doesn't set the content type header (so few people really understand HTTP, /lament/)

der.hans wrote:
> moin moin,
> 
> As I posted last week, ATI's new cards are releasing with X drivers.
> 
> http://www.phoronix.com/scan.php?page=article&item=amd_evolution&num=1
> 
> This won't satisfy some of us as they're binary blobs, but the Free 
> Software drivers can be used as well.
> 
> http://www.phoronix.com/scan.php?page=article&item=amd_rv770_oss&num=1
> 
> Now they're available.
> 
> http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Description=HD+4850&cm_sp=HomepageSL-_-AMD4000-_-http%3a%2f%2fpromotions.newegg.com%2famd%2f4000%2f270x120.gif
> 
> Hmm, why does it think that page is a gif?
> 
> Anyway, I'm buying a couple of SDHC cards, so figured I'd pick up a new
> video card as well. I no longer know squat about video cards, so I don't
> know how to rate the available cards.
> 
> I don't game, but I do want to have good compiz and video playback. I have
> a dual-head setup with 22" wide monitors as well.
> 
> I would like something that will run cool at low power. That probably
> means I should completely ignore this line :).
> 
> Video out would be cool. HD out with Free Software support would be
> awesome.
> 
>         product: M2N4-SLI
>         vendor: ASUSTeK Computer INC.
> 
> It's got a bunch of pci slots, but I don't see anything about agp or
> pciexpress in lshw.
> 
> ciao,
> 
> der.hans