With the P4B266 you can lock PCI/AGP but the P4S533 has great features like onboard LAN, C-media 6 channel sound and it's cheaper, though not by much. Ahh by the way, many P4B266 have problems with USB when overclocked so if you have USB devices, you take a guess here. I went the P4S533 route because they didn't have P4B266-C in stock and I can say it's a pretty solid board. With the P4B, I could have gone higher because of the fixed AGP/PCI but I think 2.45ghz rock solid is still pretty good. If you have a bit more to spend on a motherboard, I'd recommend the P4B533-V though, it's based on the i845G, has onboard video (disable it if you don't need it) and it's the fastest board around!