2560x1600 resolution

Hi all,

I'm looking for the best video card solution for 2560x1600 resolution. I've been reading up on SLI/Crossfire, and I understand high resolution is where multi-GPU really shines. I'm using a single Nvidia 8800GTS 640mb at the moment, and it performs ok, but I'm really looking for some outrageous performance. Here are the details:

Nvidia 680i board
Intel Core 2 6420 @ 2.13
4gb RAM
Vista Ultimate 32bit
Three 500gb HDD in RAID5
550 watt PSU
Antec Sonata II case

Here are my questions:

-To SLI or not? Would a new GTX 280 get the job done? Is it worth the extra expense for SLI?
-I'm running two monitors, one 30" and a 22." I've read SLI will only drive one monitor, do any of you have experience adding a 3rd card just for the second monitor?
- 550 watt PSU ain't gonna do it. How big do I need?
-Temperature. I have adequate cooling right now, how much more heat am I going to be dealing with?
-Noise. This thing is gonna be loud, do you have any suggestions on how I can cut down the racket?

Appreciate the help,
Mike