The chipset piece of the equation
CrossFire is more than just a dual-graphics technology, though, according to ATI. It's a platform, and that platform is anchored by the CrossFire Editions of ATI's Radeon Xpress 200 chipset. We first reviewed the AMD version of the Radeon Xpress 200 nearly a year ago, and found it to be a decent solution, despite a few warts.

The CrossFire Edition's south bridge is a newer revision than the one we first tested. Dubbed the SB450, this new south bridge includes support for Intel's High Definition Audio specification, bringing better resolutions and sample rates to stock PC audio. Unfortunately, the SB450 is missing a number of other enthusiast-class checkmarks, including newer Serial ATA features like 3Gbps transfer rates and Native Command Queuing. The SB450's four SATA ports do support RAID, but only levels 0 and 1.

A block diagram of the Radeon Xpress 200 CrossFire Edition chipset for AMD. (Source: ATI.)

Rather than fret over the Radeon Xpress 200's shortcomings, ATI has attempted to bolster its chipset's enthusiast credentials by designing a virtual showcase of a motherboard reference design and pushing mobo makers to manufacture boards based on it. We recently reviewed a motherboard from Sapphire that's very similar to the design of the ATI CrossFire reference board, with the obvious exception of the second PCI-E slot. That board overclocked exceptionally well for us and generally performed on par with an nForce4-based competitor. We did find some performance pitfalls, though. We'll revisit those in our evaluation of the CrossFire platform.

ATI's Radeon Xpress 200 CrossFire Edition reference board with dual X850 XT cards

In its reference design, ATI has augmented the SB450 cyborg-style by using PCI Express auxiliary I/O chips to support better SATA with 3Gbps transfer rates, NCQ, and Gigabit Ethernet. We'd prefer that all of a board's SATA ports offer the latest capabilities, and we'd like to have RAID levels 5 and 10. Still, the reference design's additions do put the Radeon Xpress 200 at the center of a credible enthusiast-class mobo.

A block diagram of the Radeon Xpress 200 CrossFire Edition chipset for Intel. (Source: ATI.)

Speaking of credible enthusiast-class hardware, Intel has been struggling to meet that standard with its Pentium 4 processors of late, but that didn't stop ATI from churning out an Intel-centric version of the Radeon Xpress 200 CrossFire Edition. Unlike the AMD version, this chipset needs to have a very solid memory controller on the north bridge in order to keep up with the nForce SLI Intel Edition and the Intel 955X. ATI says its Intel CrossFire chipset will support front-side bus speeds up to 1066MHz and DDR2 memory up to 866MHz, with higher RAM speeds in validation now. We'll have to test this chipset to see whether it passes muster compared to the competition, but we have reservations about the performance benefits of dual-graphics motherboards with current Intel Pentium 4/D processors.

Oddly enough, ATI will be supplying CrossFire Edition chipsets that also have an integrated graphics processor, as the block diagram for the Intel-oriented chipset above shows. These chipsets will be capable of driving an additional display even when both PCI-E graphics slots are occupied, thanks to ATI's SurroundView capability.

Another possible option for CrossFire chipsets comes from Intel in the form of the 955X chipset. ATI has announced that it will validate the 955X for use with CrossFire, but unfortunately, we were not able to obtain drivers from ATI that would allow us to test CrossFire performance with the 955X.

A new wrinkle: the transposer card
Although ATI initially claimed that no redirector card would be needed in order to switch CrossFire from single-slot to dual-slot configurations, that turns out not to be the case. Our CrossFire review kit came with a so-called transposer card that plugs into the secondary PCI Express graphics slot in order to enable sixteen-lane operation of the primary slot with only one graphics card installed. This card works in concert with a BIOS setting that shifts the board from single- to dual-slot operation.

Alternately, with two graphics cards installed and the proper BIOS settings, eight of the sixteen lanes connected to the primary graphics slot can be redirected to the secondary slot.

The transposer card rests in the secondary PCI Express slot

Don't forget to remove the transposer card from the secondary PCI-E slot when the mobo's BIOS is set to dual-graphics mode, or the board won't POST.

This card turns out to be even more of an inconvenience than the paddle card on an older or lower-priced nForce4 SLI motherboard, if for no other reason than that there's nowhere on the motherboard to store it when it's not in use. The thing looks easy to lose to me.

Crossfire's competition?
ATI has been very careful not to position CrossFire head to head against NVIDIA's new GeForce 7800 GTX graphics chips. Instead, they gingerly point out that a CrossFire rig with a pair of Radeon X850 XT cards is best matched up, in terms of price and performance, against a GeForce 6800 Ultra SLI system. ATI says its competition for NVIDIA's $499 graphics cards is coming soon, and like the GeForce 7 series, it, too, will be a next-generation product.

Of course, we couldn't resist throwing a pair of GeForce 7-series cards into the mix, but we chose NVIDIA's GeForce 7800 GT, not the GTX. The 7800 GT isn't priced too far above the Radeon X850 XT right now, all told. Still, the 6800 Ultra may be the more appropriate comparison, because our 7800 GT cards are "overclocked in the box" variants from XFX. They are real consumer products, but they do run at slightly higher clock speeds (450/525MHz) than a stock 7800 GT (400/500MHz).