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Lovers of high-end PC graphics rejoiced at news of the recent launch of desktops with PCI Express-based chip sets that promise dramatically faster graphics throughput than today's AGP 8X standard. But a preliminary look at two of the first PCI Express cards suggests that the spec will have more impact on future graphics headroom than on immediate performance gains.
In fact, our tests showed practically no performance difference between graphics cards using the AGP 8X interface and those using PCI Express. Though PCI Express 16X supports concurrent transfers of up to 4 gigabytes per second compared with AGP 8X's 2.1 GBps of shared bandwidth, even today's most graphics-intensive PC games have yet to turn the AGP conduit into a bottleneck.
We tested boards based on graphics chips from NVidia and from ATI. NVidia sent us an early PCI Express reference board using its GeForce 6800 GT chip and 256MB of memory; ATI provided a preproduction version of its board using the Radeon X600 XT chip and 128MB of memory. Retail cards based on the NVidia GeForce 6800 GT chip should be shipping by the time you read this and will sell for about $399. Cards based on ATI's Radeon X600 XT chip should be on shelves, too, selling for about $199.
We used two AGP comparison boards. One was Leadtek's $400 shipping WinFast A400 GT board with the NVidia GeForce 6800 GT chip (the same chip that the NVidia PCI Express board uses) and 128MB of memory. The other was VisionTek's $200 shipping Xtasy 9600 XT, which is based on ATI's Radeon 9600 XT chip (a chip different from but comparable to the ATI PCI Express board's X600 XT) and has 256MB of memory. Ideally, each of our test cards would have included the same memory allotment, but supply constraints left us with mismatched amounts of memory.
Comparing the AGP graphics cards with their newer PCI Express cousins proved dicey because there's no way to test both kinds of boards in the same PC (though at least one chip set vendor is contemplating producing a dual AGP/PCI Express product). Instead, the PC World Test Center matched comparably configured systems from the same vendor: one using Intel's PCI Express-based 915G chip set, and another using Intel's 865G chip set with 8X AGP.
Since the ATI and NVidia chips are geared toward different buyers (the NVidia GeForce 6800 GT is a higher-end chip), we compared only ATI with ATI and NVidia with NVidia. The two companies also differ in their method of implementing PCI Express. ATI chose a "native" implementation tailored to PCI Express from the ground up, while NVidia's early products (including the 6800 GT) use a bridge technology between the PCI Express interface and the company's existing AGP-based chips. Each company claims its approach makes more sense; our tests--using preproduction drivers--do not permit us to resolve that matter conclusively.
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