If you want a glorious desktop with two large monitors, it may make sense for you to use two 27″ iMacs, with one configured in Target Display Mode.
With Target Display mode, you can use your 27-inch iMac with Mac OS X as an external display. Connect any computer or other device with a Mini DisplayPort to your 27-inch iMac using a Mini DisplayPort to Mini DisplayPort cable or using a converter that converts other electrical, video, and audio protocols from another source device to Mini DisplayPort compliant signals.
Here in Australia, the 27″ iMac base model retails for $2068. This is only $570 more than the 24″ Apple LED Cinema Display. Contrast this to the Apple 30″ Cinema HD Display for $2798, and the iMac has a built-in computer. Get the 27″ iMac instead and use it as an Xgrid render node, use it as a NAS, use it as a Time Machine disk, use it as an Xcode distributed compiler or just keep it as a spare computer if the main one goes down. The only downside I can see to this is the power consumption of having a second computer running as you can’t use the iMac as a display without having it turned on (well, obviously) but that means that the computer behind the LCD is powered up and running. You can idle the CPU and spin down the hard drive to save power, but a standalone display will still use less power.
With the Quad-core i7 iMac beating the Mac Pro in just about all benchmarks at the moment, you can get one base model 27″ iMac as a display and the Quad-core i7 iMac with 8GB of RAM for just over $5k. To contrast, a Mac Pro Quad-core with 8GB and two 30″ displays will set you back more than $10.5k…
Here’s the lowdown from Apple on Using a 27-inch iMac as an external display.