How does it all fit together.
The nuts and bolts: how it all fits togetherATI and NVIDIA can successfully argue a technical case for extravagant multi-GPU solutions, based on scaling and image-quality gains. The business case is somewhat more convoluted, though, and sensible economics indicates that three- and four-way CrossFire needs to be extremely flexible in its application.
For the sake of clarity, we'll refer to CrossFireX as the nomenclature for three- and four-GPU operation. As such, it will, driver-permitting, work on AMD's 790FX chipset and a limited number of Intel-based chipsets that feature the requisite number of x16 (mechanical) PCIe slots - most notably the X38 and X48.
With the assumption that you have the necessary supporting motherboard and soon-to-be-released driver in place, CrossFireX is only open to the various SKUs in the Radeon HD 3800 series, which currently incorporate the Radeon HD 3850, HD 3870, and HD 3870 X2 GPUs.
You can team any combination of GPUs to make up CrossFireX. That means, for example, two Radeon HD 3850s and a Radeon HD 3870 for three-way operation. The Radeon HD 3870 X2 literally counts as two GPUs, so you can add another for four-GPU CrossFireX or, say, a single Radeon HD 3870 for three-GPU fun. Easy as pie, really.
The important point to note is that they're all based on the same underlying architecture and differ with respect to clock-speeds, number of GPUs, and on-board frame buffers.
The ideal combination would be to harness cards from the same family such that clocks and frame buffers are identical. Note, though, that they don't have to be from the same manufacturer.
So what happens when you hook up a Radeon HD 3870 X2 and a Radeon HD 3870? Due to the inefficiency caused by disparate engine clocks - 825MHz for the 3870 X2 and 777MHz for the single-GPU 3870 - the CrossFireX graphics sub-system runs at the speed of the slowest core in the setup. That's not much of a problem when grouping 3870 cards together but presents more of an issue when teaming, say, a Radeon HD 3870 and 3850 in tandem. Of course, you could always overclock the 3850 core to near-3870 speeds.
Asymmetric memory-clock considerations aren't as exacting , though, and each card's frame buffer operates at its native speed, which can range from an effective 1,658MHz for the HD 3850 to 2,252MHz for the single-GPU HD 3870.
The entire sub-system can be overclocked via ATI's CATALYST driver-panel setting though per-card overclocking isn't natively supported - it simply wouldn't make sense when the GPUs work best at a single (sub-system-wide) speed.
Having numerous cards in one system also paves the way for esoteric multi-monitor support - up to eight displays from four cards. Noting that linking the cards together in CrossFireX makes them appear as one to the operating system, you'll be limited to just two outputs when in multi-GPU (CrossFire) rendering mode.
