The 27-inch iMac can actually accommodate two external dual-link DVI displays, each with a resolution of up to 2,560 by 1,600, creating a three-headed dream machine that still has room to connect to the next generation of ultrafast Thunderbolt peripherals. This isn't just new territory for the iMac, but for all desktop PCs outside the workstation class.
Thunderbolt iMac: Surprising performance The new iMac lineup features quad-core Intel second-generation Core i5 ("Sandy Bridge") desktop CPUs and AMD Radeon HD 6000M-series discrete GPUs across the board. The fact that all models share 6MB of L3 cache, 1,333MHz DDR3 RAM, and 7,200-rpm hard drives narrows the performance gap between iMac models enough that most buyers can simply shop for the display size that suits them.
iMac upgraders, as well as users of PCs with first-generation Core CPUs, will definitely feel the kick. Sandy Bridge's on-chip memory controller, roomy cache, and faster RAM contribute to Apple's claimed 1.7X performance boost over prior-generation iMacs, and that technology is common to all configurations. Even the bargain hunters get a ride on the bullet train.
Apple realizes that for some, fast isn't fast enough. A Core i7 CPU upgrade is available as an affordable configure-to-order option for both 21.5- and 27-inch models, bumping top clock speeds to 2.8GHz and 3.4GHz, respectively. Developers, creative pros, sci-tech workers, people who run Windows in virtualization, and others who routinely max out their multicore CPUs but lack the budget for a Mac Pro finally have a machine of their own. The iMac's Core i7 upgrades not only kick up the clock speed, but also expand L3 cache and add Hyper-Threading, likely adding a 20 to 30% speed bonus over Core i5. That's a meaty upgrade for $200.
The AMD Radeon HD 6000M-series GPUs that Apple chose for iMac are uncommonly powerful for mainstream desktops, rendering iMac's lack of internal expansion slots into a non-issue for most buyers. The 21.5-inch iMac incorporates either the AMD Radeon HD 6750M or 6770M with 512MB of dedicated GDDR5 video memory. The entry-level 27-inch iMac also has a Radeon HD 6770M with 512MB of VRAM, while the top-end 27-inch model (the model I reviewed) has a Radeon HD 6970M with 1GB of VRAM (upgradable to 2GB).
These GPUs don't transform iMac into a workstation (to keep a lid on cost, space, power, and heat, iMac uses mobile GPUs) but Apple has selected the market's best. These AMD mobile GPUs compare favorably with AMD Radeon HD 5000-series desktop graphics cards, which continue to be sought after by PC gamers. If you're currently an iMac user, Apple says you should expect as much as a threefold hike in graphics performance. If you're coming to iMac from Mac Mini, MacBook, or a mainstream PC that uses integrated graphics, iMac's GPU will take your breath away.