Just did some quick searching and hear is what I found.
Info on the P4B266-C thanks to Newegg.com:
The Chipset Detail Specification is provided for your reference only.Please note that Some Mother Board Manufacturers may not choose toimplement all functions of the chipsets onto the Mother Board. Some mother board manufacturers may design mother board that exceeds the specification of the Chipset.
Asus P4B266-C Intel 845D chipset ATX Motherboard-OEM Supports Socket 478 for FC-PGA2 Pentium® 4 (Willamette/Northwood) 1.4GHz to 2.4GHz+. FSB: 400 MHz. Memory: 2 x DDR DIMM Sockets, Max. 2GB unbuffered PC2100/PC1600 ECC/non-ECC DDR SDRAM Memory. 6 PCI slots, 1 x AGP. Up to 4 USB. NO AUDIO. NO LAN.
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North Bridge
Chipset Name:
Intel 845D
Manufacturer: Intel
AGP FSB: 4X/2X 1.5 V only
CPU Type: 423/478 pin P4
CPU FSB: 400(100)
Memory Type: DDR
Memory FSB: 200/266
Max Memory: 3G
Sourth Bridge
Chipset Name:
82801BA (ICH2)
Manufacturer:
Intel
Number Of USB: 4
Version Of USB: 1.1
Support IDE Data: ATA 33/66/100
AMR/CNR/ACR* : CNR
*AMR - Audio Modem Riser
*CNR - Communication Network Riser
*ACR - Advanced Communication Riser
The North Bridge is the main portion of the motherboard chipset and incorporates the interface between the processor, AGP, PCI bus and memory controller.
The South Bridge is a bridge between PCI, ISA, and some of lower speed component which normally contain Floppy/IDE , USB, Com port, printer port, keyboard, and mouse controller.
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Info on the Non -C P4B266 courtesy of Comp-U-Plus:
Description:
The ASUS P4B266 and the Intel 845D chipset unlock the value of Pentium 4 technology with high-bandwidth DDR SDRAM. Supporting up to 2GB of PC2100/PC1600 and a peak bandwidth of 2.1GB/s, the P4B266 enables enhanced performance in multimedia applications, video, audio, 3D games and more! More Info & Product Specification
More Info & Product Specification
Processor: Socket 478 for FC-PGA2 Pentium® 4 (Willamette/Northwood) 1.4GHz to 2.4GHz+;
Chipset: North Bridge: Intel® 82845D Memory Controller Hub (Brookdale), South Bridge: Intel® 82801BA I/O Controller Hub 2 (ICH2);
FSB: 400 MHz;
Memory: 3 x DDR DIMM Sockets, Max. 2GB unbuffered PC2100/PC1600 ECC/non-ECC DDR SDRAM Memory;
Expansion Slots: 1 x AGP 4X, 6 x PCI, 1 x CNR;
VGA: AGP 4X Slot;
Audio (Optional): C-Media CMI8738 6-Channel Audio Controller;
Hardware Monitoring: ASUS ASIC BACH;
Special Features:
* USB 2.0 support (optional);
* ASUS® POST ReporterTM for system voice warnings;
* Smart Card Reader Support;
* Memory Stick and Secure Digital Reader Support;
* ASUS® MyLogoTM for customized boot logos;
* ASUS® EZ PlugTM for flexible power connectivity;
* ASUS® JumperFreeTM S/PDIF in/out interface (Optional);
* SFS (Stepless Frequency Selection);
* Vcore & Vio adjustable;
Internal I/O Interfaces:
* CD Audio-In Connector (Optional);
* AUX Audio-In Connector (Optioanl);
* Voice Modem Connector (Optional);
* USB 1.1 connector to Support optional 2 USB Ports;
* USB 2.0 connector to Support optional 4 USB Ports;
* CPU/SPS/Chassis FAN connectors;
* ATX Power Connector;
* IDE LED Connector;
* Front MIC & Headphone connectors;
* Smart Card / Memory Stick / SD Reader connectors;
Back Panel I/O: 2 x Serial Ports, 1 x Parallel Port (Optional), PS/2 Keyboard and Mouse Connector, Game/Mic-in/Line-in/Line-out Ports, 2 USB 1.1 Ports;
BIOS Feature: 2Mb Flash ROM, Award BIOS, PnP, DMI2.0, WfM2.0, JumperFree;
Form Factor: ATX 12" x 9.4" (30.5cm x 23.0cm).
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Basically I guess the main differences are 3 DIMMS Compared to 2 DIMMS (I belive with 3 DIMMS you must have single-sided memory in DIMMS 2 and 3 for them to function properly), USB 2.0, On-Board Audio, and LAN. I picked up the CDW board that is advertised with USB 2.0, but for the price I'd be happy if it didn't. (In all likelihood it doesn't cause there seems to be something wrong with the ad.)