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NVIDIA's GeForce 6600 GT AGP graphics card

Bridging backwards

WHILE WE WERE extremely impressed with the GeForce 6600 GT when it was launched back in September, the card's PCI Express X16 interface left us longing for an AGP flavor. It's wasn't that we didn't like PCI Express, but the only commercially available PCI Express platform was saddled with the Pentium 4's comparatively poor gaming performance. Since September, we've previewed Athlon 64 chipsets with PCI Express support from ATI, NVIDIA, and VIA, but none are available in production motherboards just yet. You'll still need an LGA775 Pentium 4 or Celeron processor if you want to get in on PCI Express graphics.

Fortunately, you don't need PCI Express to get in on the GeForce 6600 GT, at least not anymore. With a little help from its High Speed Interconnect (HSI) bridge chip, NVIDIA has brought the GeForce 6600 GT to AGP. Was anything lost in the translation? Read on as we explore the GeForce 6600 GT AGP's performance against its PCI Express counterpart and mid-range competition.

The GeForce 6600 GT AGP
Before we get into the GeForce 6600 GT AGP, I suggest you read through our review of the GeForce 6600 GT for PCI Express. The two cards are quite similar, except where they're not. I'll be going over the key differences between the two.

The NVIDIA GeForce 6600 GT AGP reference card

To get things started, let me point out some physical features that differentiate the 6600 GT AGP from its PCI Express counterpart. First, note that the GeForce 6600 GT AGP has an auxiliary power connector. This auxiliary power connector isn't required on the GeForce 6600 GT, which can draw up to 75W from a PCI Express X16 graphics slot. The PCI Express GeForce 6600 GT can also be teamed with a second card using NVIDIA's SLI technology. However, since SLI requires PCI Express, you won't find an SLI connector on the GeForce 6600 GT AGP. Motherboards aren't available with dual AGP slots, anyway.

Removing the GeForce 6600 GT AGP's heat sinks reveals even more about the card. The fact that there are two heat sinks to remove is also telling.

The GeForce 6600 GT GPU and HSI bridge chip

The GeForce 6600 GT AGP has two heat sinks because there are actually two NVIDIA chips on board. The first is the GeForce 6600 GT GPU, otherwise known as NV43. The second is NVIDIA's High-Speed Interconnect (HSI) bridge chip, which translates between AGP and PCI Express. NVIDIA currently uses the HSI bridge to make its graphics chips with AGP interfaces work on PCI-E motherboards. The GeForce PCX line of cards use the bridge chip in this way. The NV43, however, already has a built-in PCI Express interface, so for the AGP version of the GeForce 6600 GT, NVIDIA is turning the HSI chip around and using it to bridge between the PCI-E graphics chip and an AGP motherboard.

As far as pixel pipelines, shaders, and core clock speeds are concerned, the GeForce 6600 GT AGP is identical to the GeForce 6600 GT. The two differ when it comes to memory clock speeds, though. Both cards have a 128-bit memory bus and GDDR3 memory, but at 450MHz, the GeForce 6600 GT AGP's memory is clocked 50MHz lower than the PCI Express version of the card. That 50MHz deficit translates to 1.6GB/sec less memory bandwidth for the GeForce 6600 GT AGP, which could make the card slightly slower than its PCI Express counterpart.