I can't say for sure about the overclocking ability of a Celeron 766, but since this is a 66mhz FSB native CPU the multiplier is quite high. You probably would be *very* lucky to get to 100mhz FSB (I'm just guessing, maybe with the newer steppings this might be possible)
If your not thinking of overclocking, you'd be far better off to spend the few extra bucks to get a 800+ mhz Celeron which runs at 100mhz FSB. Overclocking these baby's on a BX system is limited to the ability of your agp card at higher bus frequency, and the motherboard to support the right PCI divider.
The Asus slocket is good. I've also used the Iwill. I've also used a Generic one without any problem.
The one thing I've found from my experience with the Celeron 566@850 on a BH6 is that the voltage adjustment jumpers found on some slockets are ignored, you have to use the bios vcore adjustments (this is probably specific to the Abit BH6 only). What this means is that it may not be worth it to spend the extra bucks to get a brand name slocket with voltage adjustment jumpers....